Goldfish and Aquarium Board Articles
Fish Medical Guide: Diagnosing Symptoms

By the GAB moderators

It is frustrating when our fish are not healthy, however, there are things we can do to help them. This article addresses some of the more commonly seen symptoms of illness in freshwater fish kept in aquariums. The table is organized by symptom. The links will take you to more information about each illness and its treatment. This article covers some of the more common illnesses and is by no means a complete listing, however, it should help you gain more understanding of common symptoms. At GAB we love to help, so stop by our Sick Fish Forum and tell us what's going on.

Behavior and Physical Changes
Bent Spine
Cloudy Eyes
Eyes Popping Out
Flashing or Rubbing
Floating or Bottom Sitting


Losing Color
Losing Weight
Pale Gills or Other Gill Abnormalities
Scales Sticking Out (Dropsy)
Slimecoat Changes
Spitting Out Food
Spots, growths, bumps and fuzz
Holes or Craters
Poop: White and Stringy


Spots White


Yellow, Gold, Brown
Raised Brown or Dark Green
Red Streaks, Stripes or Blotches
Rot or Deterioration of Fins
Worms Black on Fish
Red from Vent


General considerations when treating sick fish:
When your fish aren't feeling well, the first thing you should do is check your water quality and take steps to resolve any water quality problems. Good water quality is the most important thing you can do to help keep your fish healthy. Second, try to identify specifically what the illness is based on symptoms. Third, choose a treatment based on your diagnosis and understand how that treatment works.

To be prepared for illnesses, make sure you have all the the necessary water test kits ready. You may also want to stock your medicine cabinet so that you have the things you need when your aquatic friends get sick. If you are unfamiliar with fish anatomy, make sure to check our external and internal anatomy articles. When treating with medications, you should be up to speed on your fish, as some species of fish don't tolerate certain medications well (e.g. Scaleless fish and salt, tetras and Formalin/Malachite Green). To further familiarize yourself with medications read GAB Articles on Medications and Medicated Food. As you can see when you visit our GAB Sick Fish Forum, we believe in assessing the situation and finding the most appropriate solution when faced with fish problems. You may also want to check if there is an aquatic vet near you.

When you medicate, you usually want to follow the directions on your medication, turn off UV sterilizers, and remove carbon from your filter before treating with waterborne medications. It is also good to do a partial water change with a thorough vacuuming prior to beginning treatment. Depending on the situation, it may be best to treat in a hospital tank. Also remember that you can try to minimze the possibility of diseases by buying specimens that look healthy, quarantining them and feeding them good nutrious food.



Definition: Flashing means that the fish is throwing itself against or rubbing against decorations, gravel or tank walls. It is a sign of discomfort and irritation. This irritation is likely felt externally so you are looking for external answers. Poor water quality or parasites is the usual explanation.
Causes and Remedies
Water Quality

If your fish is flashing, this may be due to poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum, you should test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Irritants in the Water

In addition to ammonia and nitrite, other irritants in the water may cause flashing such as chlorine, metals, household cleaners, etc. Make sure you use a water conditioner that removes chlorine and binds heavy metals. If you suspect other chemicals, preform a large water changes and add fresh carbon to your filter. Also, if you have added decorations to your aquarium that are not aquarium safe, they may leach chemicals into your water. Remove the decoration, perform a large waterchange and add fresh carbon to your filter.

Parasites Flashing may be a sign of external parasites, check the fish carefully for other symptoms such as slimecoat changes, spots or anchor worms. After you rule out water quality and the fish continue flashing, consider treating the tank for parasites with a product containing formalin and malachite green (e.g. Rid Ich or Quick Cure ). If it is a goldfish or other salt tolerant fish, you can treat with salt as described here, however, there are salt-resistant species of costia that may not clear with salt. Goldfish are susceptible to flukes. If you have a microscope, you may want to scrape and scope to determine whether flukes are present, or if that is not possible, treat for flukes with praziquantel.

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Definition Gasping is when the fish are at the water surface trying to get air. This is also called piping. As with most symptoms, it is either the environment (water quality) or an illness that is causing this behavior. Piping is rather serious and you should try to identify the causes and remedy this immediately.
Causes and Remedies
Water Quality

If your fish are gasping, this may be due to poor water quality. Check your water parameters and perform water changes if necessary. These tests are done best with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Oxygen Levels If you have ruled out water quality and fish are still gasping, they may not be getting enough oxygen. Add an air stone or venturi to increase surface agitation. Vacuum your gravel to remove debris (decaying debris consumes oxygen). Lower your water level so that your filter output breaks the water surface.
Nitrite Poisoning or Brown Blood Disease If your nitrite is not 0 ppm, look at the gills. They should be bright blood red and well defined. If they are brown, this indicates brown blood disease from nitrite poisoning. Nitrite keeps red blood cells from being able to transport oxygen. Perform water changes to lower nitrite. If the fish is salt tolerant, add some salt (1 tsp per gallon) to help protect against nitrite entering via the gills. The chloride ions in salt result in the fish absorbing less nitrite trough the gills. Methylene blue can be used to unbind the red blood cells. You can read more about nitrite poisoning here.
Chlorine or Chloramine Chlorine or chloramine can also cause gasping if you did not dechlorinate your water (or if your city added more chlorine/chloramine than your water conditioner could detoxify). As with nitrite poisoning, the gills will look brown instead of red. Methylene blue can be used to unbind the red blood cells. If you suspect high chlorine or chloramine levels, you could call your city's water quality department to find out how much disinfectant they add to your water supply. If this is the cause of the problem, you need to increase the amount of dechlorinator you use.

Review the latest events and consider whether something poisonous or toxic may have gotten into the tank (e.g. sprays, soap, etc.). If that is the case, do a large water change and add fresh carbon.

Sometimes the gas H2S (hydrogen sulfide) may leach out of the substrate. Hydrogen sulfide gas is formed by anaerobic bacterial action. If you (or the fish) disturb the substrate, this gas may bubble out and cause gasping. In severe cases, H2S poisoning can cause swirling, flip-over and death. However, the gas is not that soluble in water, so the fish have to get a face-full to make them sick, especially if you have plenty of surface agitation in the tank. You should be able to identify this gas by its smell (much like rotten eggs). It is advisable to move your fish to another container with their tank water while you remove all but an inch of gravel and vacuum the tank well before returning the fish. To avoid this problem, vacuum the substrate regularly, or if you have heavily planted tanks be careful not to disturb the substrate too much.

Flukes or Other Gill Parasites

Gill flukes and other parasites or bacterial infections can damage gills. Check the fish for symptoms of bacterial or parasitic problems and treat appropriately (praziquantel for gill flukes, formalin/malachite green for other parasites or antibiotics for bacterial infections).

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Definition If you notice that the fish is not behaving normally, but instead is hanging around the top of the tank or near the bottom and is not swimming around much, the fish is probably lethargic. This is a behavioral symptom that is a sure sign that something is wrong. Once you observe that a fish has become lethargic and after you've checked your water parameters, you have to look for other symptoms since lethargy is not specific enough to provide a definitive diagnosis.
Picture This lethargic goldfish is spending an unusual amount of time in the bottom corner of the tank. You can also tell he is not feeling well since his dorsal fin is not erect.
Causes and Remedies
Water Quality

If your fish is lethargic, this may be due to poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own liquid test kits. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Temperature Problem If the fish is not moving around much, it may be because the water is either too warm or too cold. Take care to measure the temperature. Tropical fish like a temperature around 78-79 F. For many tropical fish anything lower than 77 F may slow them and cause them to become stressed. Goldfish will do well in temperatures from 68 to 79 F. Most aquarium fish can tolerate temperatures up to 82-84 F, if you provide plenty of airation (warmer water holds less oxygen). To minimize stress, change the temperature of the water in your tank slowly/gradually.
Infection If you fish is lethargic, check for other signs like redness, fuzz or fin deterioration that would suggest bacterial causes. If you cannot find any such signs (and you have ruled out water quality, parasites, egglaying, or dropping fry), check to see if the fish is still eating. If it is lethargic and not eating, it is probably an internal infection and it would be prudent to move the fish to a hospital tank. Feed antibiotic food (like Medi-Gold) or if the fish is not eating, treat with a broad spectrum in-tank antibiotic like kanamycin. If the fish is big enough you should consider injections. Keep checking for other signs of illness.

Parasites can cause lethargy - particularly parasites that attack gills such as flukes or costia. Check carefully for signs of these parasites by looking at the slimecoat and seeing if the fish is gasping or flashing. Also if possible check the gills.

Lethargy may also be a sign of internal parasites. These are hard to diagnose as they do not leave many clear symptoms except lethargy, failure to thrive and losing weight despite eating. If you have access to a microscope, you can try to identify potential pathogens by looking at some poop emulsified in tank water. Treat for internal parasites with Jungle Anti-Parasite Medicated Fish Food. Read more about treating for internal parasites here.

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Sometimes we can observe fish making weird shapes with their mouths, namely forming their mouths into an 'o' shape and looking like they are yawning. An occasional 'yawn' should be of no concern, but if the fish does this repeatedly through the day there may be something bothering it. Often times fish display irritation in the gills by opening and closing the mouth rapidly.

The yawning (or mouth opening and closing) is very likely an attempt to clear the gills, therefore, those are the areas we focus on when we look for other symptoms. Try to check the gills of the fish to see it they are normal for the type of fish you have. Here are some pictures and links on gills.

Causes and Treatment
Water Quality

If your fish is yawning, this may be due to poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Yawning could also be caused by low oxygen levels, so add an air stone or venturi to increase surface agitation. Vacuum your gravel to remove debris (decaying debris consumes oxygen). Lower your water level so that your filter output breaks the water surface.

Gill Parasites

Several parasites can attack gills and cause stress to the fish.

To determine which parasite is involved, you need to look for other signs in addition to yawning. Is the fish lethargic, can you see spots or changes in the slime coat?


  • Flukes
  • Flukes (Dactylogyrus or Gyrodactylus) are parasites common in goldfish in particular. They cannot be seen with the naked eye, so you need a gill scrape or a body scrape to diagnose flukes. Here is a video of moving flukes from Koivet, and more information on flukes can be found here. However, since scraping and scooping are not always possible, we sometimes treat for flukes if several of the symptoms are present and we have no other good explanation for the symptoms. Thankfully a very benign product, praziquantel, is now widely available. If it is not available locally, you can order it (in the form of Prazi-Pro) from outlets such as the Goldfish Connection, PondRX or MOPS (in Canada). Treat as suggested here

    fluke fluke

    These are microscopic pictures of gill filaments with a gill fluke (Dactylogyrus) clearly visible.
    The fluke, as can be seen at the picture to the right, is identified by its four eye spots.

  • Protozoan Parasites
  • Protozoan parasites (such as ich or costia) should also make the fish display other symptoms (flashing, slimecoat changes, spots) so look for those. These parasites are best treated with a malachite Green/formalin combination (such as in Rid Ich or Quick Cure), or if the fish is salt tolerant, plain salt. Note: some strains of costia and ich are salt resistant. Also make sure you read the label of the medication to make sure your fish can tolerate it.

  • Hexamita
  • If doing any of the above did not help, treat the fish for Hexamita with metrodonidazole (aka Flagyl). Or if this is not available, use a parasite medication that has a shot gun approach like Jungle Parasite Guard.

Other Gill Problems There may be other gill issues. Check the gills and read about diagnosing gill problems here.

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Spitting out Food
Definition If your fish does not eat well or spits out the food, there is very likely a problem. However, there are several possibilities for what this can be so you have to observe carefully when this happens.
Causes and Treatment
Water Quality

The behaviour may mean your fish is not feeling well, and water quality may be the under laying cause so check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Fish does not like the food Some fish are very picky eaters. They will not take well to new offerings and can spit things out that feel different to them. Bettas for instances are notorious for their food preferences. Make sure that you are feeding food that fills the requirement for your species and give the fish time to adjust.
Problems with the mouth In some cases, spitting out food can mean that the mouth is sore. Observe carefully and if possible have a look inside the mouth. Some larger fish, like goldfish, may get gravel stuck in the mouth. If the fish cannot get rid of the gravel, you may have to try to remove it with tweezers. Another possibility is mouth rot - a form of the illness called flex or columnaris caused by the bacterium Flavobacterium columnaris. Read about treating for flex here.
Gill Parasites

Several parasites can attack gills and cause stress to the fish

To determine which parasite is involved, you need to look for other signs as well. Is the fish lethargic, can you see spots or changes in the slime coat?

Read more here.

Illness or Infection Sometimes a fish can be too lethargic and sick to eat because it is fighting another illness. Look for other symptoms to the help you diagnose this illness.
Mouth Brooder Some species are mouth brooders and will carry their fry in the mouth. During that period, eating will not be the priority. This is natural part of the fish's life and nothing to worry about.

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Discoloration and spots are very common symptoms. When we see spots it is important to look at them closely to find out what color they are as well as whether they are raised, smooth or fuzzy.

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White Spots
Definition If the fish is coated in small white spots that makes it look like the fish has been sprinkled with sugar or salt, the fish has a parasite called ich. However if the spots are larger and more blotchy, they are probably flex (aka columnaris).
Causes and Treatment

White spots that look like sugar coating, indicate that the fish has ich, like in this picture. Ich is the common name for the ciliated parasite Ichthyophthirius mutifillis, which has a complicated life cycle. If left untreated, ich is deadly. Treat the whole tank with Rid Ich (following the instructions on the bottle) or Quick Cure. If it is a goldfish, you can treat with salt.

Flex or Columnaris White fuzz on fins and body is most likely a bacterial illness, flex/columnaris caused by a bacteria Flavobacterium columnare (previously called Flexibacter columnare). Flex/columnaris can be a very quick killer. Fuzz is not always how flex manifests itself. Sometimes there may be redness and deteriorations or sores. Redness in the mouth area is sometimes flex. Flex may also be seen as greyness or yellowness to the slime coat of fish. It is important to isolate the fish in a hospital tank to eliminate spreading. If the fish is salt tolerant, add 1 tsp of salt per gallon of water in the hospital tank and to your main tank as well (to help prevent infection and spreading). Flex is an opportunistic infection, and mild cases will clear up with super clean water and a little salt. In advanced cases, the fish will need antibiotics. Since flex is mostly an external infection, bath antibiotics may be effective such as Maracyn Plus (sulfadimidine and trimethoprim), Jungle Fungus Clear (nitrofurazone, furazolidone, potassium dichromate), and kanamycin. Potassium permanganate is a very effective flex treatment, but it must be used with caution as it is a strong oxidizer and could put a very sick fish "over the edge." Some strains of flex can be hard to treat.
Breeding Stars

Male goldfish get breeding stars on their operculum and first ray of their pectoral fin. These are natural, so no need to worry or treat.

Read more about sexing goldfish here.

Male goldfish with breeding stars on operculum and pectoral fin
Mystery Spots on Goldfish Fins On the Goldfish and Aquarium Board forums we have seen a few goldfish with spots or small pimple-like bumps on their tails that may resemble columnaris but which do not respond to treatment with either parasite medicatons or antibiotics. They do not appear to cause the fish discomfort and seem to come and go. At this point most of us think they may be caused by a viral agent but we do not really know. One of our moderators, Ingrid, has documented the story of her moor and the Mystery Spots here. Also, Betty's pictures of Lumpy's tail are here.

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Yellow Spots or Sheen
Definition If your fish looks like it is getting a golden brownish sheen to it, you are dealing with another parasite.
Causes and Treatment

If there is a coating of yellow (or brown) on the fish, the fish has velvet. This is a parasite – a dinoflagellate called Piscinoodinium (formerly Oodinium). The fish may also have clamped fins or scratch against tank decorations or gravel (flashing). Once the attack is severe, the fish may become lethargic and you may see rapid gill movements. This parasite will attach to and attack the skin or gills of fish. It is deadly and quite contagious, so treat the whole tank with Rid Ich or Quick Cure. Sometimes velvet can be difficult to diagnose, and it may help to shine a flashlight on the fish to see the yellowish coating. Picture

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Bigger raised dark spot
Definition If you see green or brown bumps on your fish, and the fish is acting like it has parasites (flashing, clamped fins), study the bump carefully to see if it has black dots on it or if it moves.
Causes and Treatment

If you see the eye spots, your fish has a parasite called Argulus , also known as fish lice.
Argulus is a disc-shaped crustacean parasite (a copepod) that gets up to 10 mm in diameter. It will attach itself to the fish with suckers and suck blood from the fish. This is very irritating, and the fish will normally flash. Treat by removing the lice with tweezers if possible. If the removal leaves a sore, you can dab quickly with some hydrogen peroxide or iodine/betadine to prevent infection. Lice can live several days (maybe as long as 15) without a host and are egg-layers, so to make sure all lice have been removed from the tank environment, you may want to treat your tank with dimilin (found in a medication like Anchors Away). Another effective treatment is the chemical trichlorfon, found in products like Clout and Fluke Tabs. Trichlorfon is a neurotoxic pesticide and must be used with caution.

Argulus pictures

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Red Spots
Definition Red spots anywhere on the body of your fish.
Causes and Treatment

Small red spots (especially on the underside of the fish and companioned with flashing and rubbing) could be costia - the protozoan parasite Ichthyobodo. Sometimes costia will not always show up like this, but rather presents as slime coat changes and perhaps some blueness or blackness to the fish. A clear diagnosis sometimes requires a microscope. See a picture of it here and microscopic picture here. Costia should be treated with Rid Ich or Quick Cure. Though it has a short life cycle, it can be quite hard to treat and treatment requires several applications.

Internal Infection If the redness is in the form of blotches or red streaks, the fish may have an internal bacterial infection, usually caused by gram negative bacteria such as Aeromonas or Pseudomonas. Often the redness can be found at the base of the fins. Treat by feeding antibiotic food (Medi-Gold preferably) or if the fish is not eating, use a full spectrum water antibiotic like kanamycin, a combination of Maracyn (erythromycin) and Maracyn 2 (minocycline), or Maracyn Plus (sulfadimidine and trimethoprim).
Other possibility If the fish is not showing other signs of parasites, red spots may be caused by slight irritation, rubbing, or scratching and the best thing to do would be to check the water quality and just watch the spots. If the spots are open, they would be considered sores.

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Black Spots
Definition Black on the fish is often less of a concern than the other spots we see.
Causes and Treatment

Melanophore Migration
(or healing)

Black is normally an indication that the fish has suffered a chemical burn, and this burn is now healing. This is sometimes called "melanophore migration." If you see black, check your water and perform water changes to correct problems.

Goldfish with black spots caused by poor water quality. These spots disappeared when the fish recovered in water with perfect quality. Here is a picture of the recovered goldfish.
Uvulifer ambloplitis Much rarer are the black spots caused by a parasite (a flatworm called Uvulifer ambloplitis ). Since this parasite's lifecycle requires intermediate hosts (like birds), it is found only in ponds.

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Spots or craters on head or along lateral line
Definition Here is a picture of some of these craters.
Causes and Treatment
Hole in the Head Disease

These kinds of spots are associated with a disease called Hole in the Head Disease (HITH) or HLLE (Head and Lateral Line Erosion). It is a disease that affects cichlids in particular but is still not clearly understood. Several causes have been explored such as water quality, mineral or vitamin deficiency, infection or parasite infestation. (Hexamita). Treatment is therefore difficult. But we would recommend checking water, addressing food issues, feed Metro-Med (to deal with Hexamita). Read more about HITH here or here.

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Worm Attached to Fish
Definition Of the common parasites only anchor worms, leeches, and fish lice are big enough to be easily seen with the naked eye.
Causes and Treatment
Anchor Worm

If you see a worm attached to the fish with forked ends, it is an anchor worm, a parasite also known as Lernea. See picture here. Treat by removing the anchor worm with tweezers if possible. If the removal leaves a sore, you can dab it quickly with 3% hydrogen peroxide or iodine/betadine to prevent infection. Anchor worms are egg layers, so to make sure your tank is free of worms, you may want to treat your tank with dimilin found in a medication like Anchors Away.

Leeches Leeches or Hirundea are flattened, segmented worms with a sucker on both tail and mouth. They move by attaching the mouth sucker, then tail sucker, then mouth sucker, and so on, to the surface they are on. See another picture here. They can latch on to the fish and cause harm and irritation. They are not easy to remove. A salt dip is a possibility and there are some commercial remedies available. Leeches are not common in aquariums but can attack fish in ponds.

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Red worms protruding from the vent
Definition Fish can suffer from several kinds of internal parasites. This is a condition that is hard to diagnose. Often failure to thrive is the only clear indication we get. However, one common internal parasite is different in that we can observe it protruding from the vent area of the fish as shown in the picture below.
Causes and Treatment

If you can see a red worm protruding from the anus (vent), your fish has an internal parasite called Camallanus. Internal parasites can come from live food or be transmitted from fish to fish. This roundworm (nematode) is easy to identify because of its tendency to hang out the vent. Nematodes are best treated with fenbendazole, piperazine, or levimasole. Jungle Anti-Parasite Medicated Fish Food contains levimasole.

Picture of Celebes rainbowfish with Camallanus

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Fuzz, cotton growth
Definition It is not unusual to see fuzz growing on fish, and when we do, it is clear that the fish in most cases is ill. This fuzz is sometimes described as cotton wool and some people refers to fuzz as cotton wool disease. Others call all fuzz fungus. But only rarely is the fuzz or cotton growth that we can observe caused by fungi; in most cases it is caused by a bacterium.
Causes and Treatment

White fuzz on fins and body is most likely a bacterial illness, often called flex, columnaris, or cotton wool disease that is caused by a bacteria--Flavobacterium columnare (formerly called Flexibacter columnare). Columnaris can be a very quick killer, especially when it infects the gills. Flex often looks fuzzy--like cotton wool--but not always. Sometimes there may be redness and ulcers/sores. Redness in the mouth area is sometimes flex. Flex may also be seen as greyness or yellowness to the slime coat of fish. Another symptom is a saddle back lesion where the fish turns gray in the upper part of its body. It is important to isolate the fish in a hospital tank to reduce the risk of the disease spreading to other fish. Some strains of columnaris can be hard to treat. If the fish is salt tolerant, add 1 tsp of salt per gallon of water. Adding salt to your main tank will help prevent infection because it keeps flex from being able to stick to the fish. The fish will need antibiotics. Several antibiotics may be effective against columnaris - erythromycin and oxytetracycline are traditionally thought to have really good effects. Other options are sulfa drugs, nitrofuran antibiotics, and potassium permanganate.

Treatment Options

If the fish is still eating, choose one of the following options:

  1. Feed Jungle Antibiotic Food (can be combined with Jungle Fungus Clear bath treatment) or
  2. Feed Medi-Gold or Romet B (can be combined with kanamycin bath) or
  3. Feed Metro Med

If the fish is not eating or eating poorly, choose one of the following options:

  1. Add Maracyn I combined with Maracyn II to the water or
  2. Add Jungle Fungus clear to the water or
  3. Add kanamycin or kanamycin sulfate to the water

If you are an advanced fish-keeper, you may consider using potassium permanganate (PP) at the start of a treatment regimen to help knock down the bacterial load on the fish. Please consult with us on the Help! My fish is Sick! Forum or published texts for guidence on using potassium permangante. Because PP is an oxidizer, it can cause gill damage.

Pictures of Columnaris
Platy infected with Flex, You can see flex as discolouration and slimecoat changes on the front of the fish's body. The same platy with the Flavobacterium (flex) infection increasing. You can clearly see the discolouration as well as fin rot in the tail area.
True Fungus If the fish has been injured and you see fast growing white/gray fuzz, this is most likely fungus and should be treated with Maroxy (treatment for true fungus). If it is a large fish, you can skip the whole tank treatment and just dab the affected areas with iodine/betadine or malachite green.
A cory with true fungus on its pectoral fin. This fuzz was growing very fast and it is rather long and fuzzy, both indications of true fungus.
Fungal Spots on Wen Goldfish with wens may get fungal spots on their wens. Some times these spots are called 'growth spots'. Usually intervention is not needed, but occasionally if it looks bad you can clean it with some 3% hydrogen peroxide or some iodine/betadine on a Q-tip (taking care not to get peroxide or iodine in the gills or eyes). Depending on how cooperative your goldfish is, you may need to sedate it before you do this.

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Definition Red sores are best diagnosed by knowing the fish's history. If the fish has suffered trauma the red sores are actual wounds, but if the sores slowly appear they are likely caused by bacteria and are developing ulcers. Redness in the mouth area is probably flex.
Picture of goldfish with a sore This goldfish (a black and orange telescope) had a sore and redness that appeared to be bacterial. If left untreated, this sore would very likely develop into an ulcer. Goldfish with sore
Causes and Treatment

Red sores on the body are often signs of a bacterial infection and ulcers. The bacteria that causes these sores is often Aeromonas hydrophila. This type of infection is often secondary to fluke infestation, so we recommend treating for flukes as well as addressing the sores directly. It is important to isolate the fish in a hospital tank. You can treat topically as explained here and combine this treatment with feeding of medicated food. Medi-Gold and MediKoi are the best choices for treating this kind of infection. If there is a white center developing, it is clearly infected and may be ulcerating. The fish may need to be injected with an antibiotic, or if that is not possible treated in a bath of Tricide Neo (sometimes sold as Neocide 3).This is a serious illness, but treatable, so please post in our forum so we can help you more specifically.

Cuts and Scrapes If the sore is the result of an injury, the best thing to do is to keep the water clean and feed good food so that the fish will nicely recover on his own. The sore can be swabbed one time with Iodine or 3% hydrogen peroxide. Additional swabs will kill the new, healthy cells growing in. If the fish has suffered some trauma and is bleeding, pressure can be applied to the wound to stop the bleeding. You may want to sedate the fish first as described here.

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Red stripes or streaks
Definition Red streaks/stripes/veins in the fins, often the tail fin.
Causes and Treatment
Water Quality

Streak are often caused by poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum. Some people associate red streaks with either high nitrate or with low oxygen. In either case a partial water change should help.

Natural On some fish (goldfish in particular) that are very light in colour, you can see bloodlines on their tails. This seems to be just a natural thing so if keeping nitrates low (under 40) and having plenty of aeration do not help, and the fish is acting healthy otherwise, consider these streaks normal.
Bacterial Infection If attending to water is not helping and the fish is looking lethargic, it is likely a bacterial infection. Treat by feeding antibiotic food (Medi-gold preferably) or if the fish is not eating, use a full spectrum water antibiotic like kanamycin or a combination of Maracyn and Maracyn 2. If the fish is larger, you should consider injections.

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Fin rot or deterioration
Definition Fins are vulnerable to poor water quality, aggression and to bacterial problems. If you see deterioration of fins, you need to analyze the situation carefully. Many people over-treat fin problems with antibiotics, but checking and adjusting water quality or looking for aggression between the fish is the first place to start. Some fish, like bettas, may blow their fins and there is no reason to medicate but plenty of reason to see whether decorations, tank mates or environmental stress factors are the main cause of the problem.
Pictures Finrot BettaFinrot
Picture is showing redness (1),red area has sloughed off (2) and healing (3) Finrot on Betta. Only the red areas are finrot
Treatment and Causes
Water Quality

Fin rot is often caused by poor water quality Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Stress or incompatible tank mates Some fish (like Bettas) react to stress by blowing their fins (or even sometimes nip at their own fins). Stress can be caused by sudden changes in pH or water temperature. Many fish are fin nippers (like Barbs, Serpae Tetras) and will cause fin problems. However in most cases clean water and good conditions will keep up with the nipping.
Bacterial Problem If the ends are red and/or you are seeing very rapid fin loss, you may have to treat for bacteria (Aeromonas, Pseudomonas, Flavobacterium) . If the fish is bigger and you can handle it, swabbing the fins with peroxide or iodine and applying triple antibiotic cream may be best--combined with medicated food. If the fish is small, feed medicated food or use water antibiotics if it is not eating. If the fish is larger, you should consider injections.

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Eyes bulging out/Popeye
Definition Popeye means that one or both eyes bulge out abnormally. This is caused by accumulation of fluid either in the eye itself or behind the eye. Pop eye is a very complicated condition, mainly because it is a symptom of an underlying problem. Part of the challenge of treating is to try to find the underplaying cause which can be poor water quality, bacterial problems, nitrogen supersaturation (gas bubble disease) and/or nutritional deficiencies (according to Edward Noga) . Picture
Picture Goldfish with popeye
Picture This is a bloated albino cory with eyes that are clearly bulging out as well.
Causes and Treatments
Water Quality

Popeye may be caused by poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Bacterial Problems

In addition to water quality, an internal bacterial infection is often the cause. However it can be hard to clearly diagnose such an infection. The bacteria that could cause popeye may be Aeromonas, Pseudomonas, or Edwardsiella, all of which are gram-negative rod bacteria. In some cases these bacteria may also cause red blotches or red streaks on the body and fins. You may be able to spot an internal infection early if the area around the pectoral fins is red. It is when the bacterial infection attacks the internal organs that the eyes begin to collect fluid. If your fish is still eating, treat by feeding antibiotic food. The most effective medicated food is Medi-Gold, which contains three antibiotics, including kanamycin. If the fish is larger, you should consider injections.

Diet Experts have identified nutritional deficiencies as a cause for popeye. These deficiencies may also be seen as failure to thrive and other signs of starvation. It is important to pay attention to the nutritional needs of the fish and to feed a varied diet.
Gas Bubble Disease

In rare cases, excess levels (super saturation) of nitrogen in the water may cause gas bubbles in the blood. These gas bubbles can collect behind the eyes causing the popeye. To prevent this make sure there is plenty of water agitation as you add the water and use air stones. You can read more about supersaturation here or here.


Dropsy Sometimes popeye is the first sign of a serious bacterial illness that will lead to dropsy. The fish is retaining fluids, causing it to bloat and get raised scales that make the fish look like a pine cone. It is very hard to save a fish once this happens. Most cases are fatal. Goldfish in a few cases have responded well to Metro-Med, so if the fish is eating this is worth trying. Other have tried antibiotic injections to try fight bacteria.
Other Causes

Experts (such as Edward Noga) have identified parasites and viruses as possible causes of popeye. Certainly none of the common parasites in fish tanks will cause popeye so these are much rarer cases.

Doc Johnson from Koivet has identified Fish TB as one possible cause of popeye .

There are times when injuries and trauma may have the effect of making the eyes pop out or in some cases a tumor growing behind the eyes.

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Cloudy Eyes
Definition Eyes can be described as cloudy if the outer covering is milky-colored or opaque or if the fluid inside the eyeball is cloudy. Sometimes there may also be a cloudy fungus film on the outside of the eye.
Goldfish with cloudyness that was outside likely from poor water quality Angel with cloudyness inside likely a bacterial problem.
Causes and Treatments
Water Quality

If your fish develop cloudy eyes, the most likely reason is poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Bacterial Illness It is possible that bacteria may cause cloudy eyes. Look for other signs such as loss of appetite, bottom sitting, redness at the base of the fins, or popeye. If the fish is not eating, use water antibiotics. If the fish is larger, you should consider injections.
Injury Quite often cloudy eyes are caused by trauma such as netting, handling, or injury on sharp objects in the tank. Make sure you are as gentle as possible if you must handle fish and take steps to remove sharp objects that the fish may injure themselves on.
Old Age Some fish may turn blind with old age. Good nutrition and healthy tank conditions may help the fish keep its sight well into old age.
Eye flukes These are not very common in aquariums. The most common eyeflukes (Diplostomum spp.) require birds as part of their life cycle.

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Slimecoat changes

Fish have a slimecoat (a mucoprotein protective layer) that covers their scales . This coat provides protection against cuts, scrapes, and invasive pathogens such as parasites and bacteria. The enzymes and antibodies in the slimecoat helps kill invading organisms, so the slimecoat is truly the armour of our fish.

Because of the slimecoat it is important to handle the fish as little as possible, and when we have to handle them, be as careful as possible. At GAB we do not believe in adding any of the products that claim to help strengthen slimecoat. There is little evidence that such products do much good. We worry about what 'unnatural aquarium products' like aloe vera may do to the gills of fish. An exception to this may be products that contain vitamin B12, which may help to strengthen the fishes own ability to produce slimecoat.

Fish can develop excess slimecoat. This is an immune response and usually occurs because something is irritating their skin. Excess slime can either be seen with the naked eye (sometimes even in the form of stringy mucus hanging off the fish), or you can check for excess slimecoat by touching or by running a popsicle stick (wooden or plastic) gently along the side of the fish.

Causes and Treatments
Water Quality

Excess slimecoat is very often caused by poor water quality. Poor water may cause the fish to strengthen its defense system. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

A change in pH causes stress and often changes to slimecoat, particularly in goldfish. If you see grey slimecoat, pH is the first thing you ought to check.

Salt levels For most fresh water fish, high levels of salinity in the water can cause irritation and lead to excess slimecoat production. Some people believe that by adding salt and causing the fish to produce this excessive slime coat they are helping the fish ward off pathogens and stay healthy. However the effect is likely counter productive as the fish is stressed when it produces more protective slime, and stress negatively affects both osmoregulation and immune response. Some fish (like goldfish) can tolerate low levels of salt (up to 0.3%) very well and salt can be used to cure some parasites like ich.

One of the major reasons fish produce excess slimecoat is in response to external parasites such as ich, costia, and flukes.

If you can not see other signs of parasites (see symptoms on spots and flashing) and the water is fine but the fish is producing excess slimecoat, you may want to treat with Quick Cure or Rid Ich, as the cause is likely the parasite costia.

Read here about Bugsy the black moor's costia. Includes good pictures of scopes of the parasite.

Other irritants

It is possible for excess slimecoat to be caused by household irritants such as sprays and cleaners that may enter the water. Make sure that you do not use any cleaners near the aquarium and that your hands are clean if you stick them in the water. Adding fresh carbon will help remove chemicals that may have entered the water.

Sometimes fish may be irritated because they are rubbing against ornaments or decorations in the tank constantly. So make sure you plan for plenty of swimming room for your fish.

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Gills: Paleness or other problems

Gills are vital organs as they are responsible for respiration, excretion of wastes, and they have a role in maintaining fluid and electrolyte balance. Gills are made up of threadlike structures called filaments. Each gill filament has secondary lamellae protruding off of it like a christmas tree with branches. The epithelium covering these structures is only one or two cells thick. The gill filaments contain a network of capillaries that result in a large surface area in contact with the water. Because they are so vascular, gills are susceptible to damage from water quality and are targets for both bacteria and parasites that can cause severe problems for the fish. Symptoms associated with gill damage include oxygen seeking behavior (e.g. rapid gill movements, flared gill covers, piping or gasping at the water surface), excess mucus, lethargy, lack of appetite, or irritation (e.g. rapidly opening the mouth and fluffing the gills). When fish appear to be sick, looking at the their gills may provide clues as to what's going on. Healthy gills should be blood red (or cherry red) in most fish. If the fish is big enough, you can gently open the gillcover and inspect the gills.

Picture of Healthy Gills Healthy gills in most fish should have a bright blood-red colour and the filaments should be uniform and well defined. When you look at them, you should be able to see the filaments clearly. Signs of problems include: brown or pale gills, blotches or stripes, or the gill filaments are not clearly divided (clumped together) or are worn or torn at the ends. Healthy Gills on Goldfish
Picture of Gills This picture gives a good indication of how to look at gills in a bigger goldfish. The goldfish is held over water and partially in the water. The picture also shows off the nice dark red colour that goldfish gills should have when healthy.
Different Gill Abnormalities and Possible Causes
Pale Gills

If the gills are pale, the fish may be experiencing anemia or organ failure. If the gills are pale and the fish has red streaks on its body or fins, is bloated or dropsying, organ failure is very likely. Paleness may also be caused by blood loss caused by parasites like flukes attacking the gills. If possible, look at a mucus scrap under the microscope and attempt to determine whether parasites are involved. Flukes are common in goldfish, and can be treated with praziquantel. For other external parasites a malachite green/formalin combination is usually effective. Read more about gill parasites here. If the fish is very weak and organ failure is suspected, you may consider euthanasia.

Brown Gills
Brown Blood Disease

If the gills are tan or brown, this is likely 'brown blood disease.' In normal blood, hemoglobin is responsible for the red color. Hemoglobin carries oxygen in the blood. when fish are exposed to nitrIte or chlorine, these cause hemoglobin to be converted to methemoglobin which can not carry oxygen, so the fish can suffocate. Most larger cities are now using chloramine to disinfect drinking water, so letting the water sit out over night will not make it fish safe. Be sure to use a dechlorinator if you use city water.

If your fish have brown gills, the first thing to do is to test your water for nitrite and perform water changes as needed to reduce nitrite levels. To protect against nitrite poisoning, if the fish is salt tolerant, you can add salt to the water at 1 teaspoon per gallon of water. Salt protects against nitrite poisoning in some fish (with chloride cells in their gills), because the chloride competes with nitrite for uptake via the gills. Methylene blue can be used in cases of nitrite poisoning to unbind the red blood cells. If you are seeing nitrite, you should re-evaluate your stocking and filtration as nitrite should not be present in established tanks.

Filaments 'stuck' together

Hyperplasia (excessive cell proliferation) occurs in response to irritation. This excess cell growth is typically combined with excess mucus production causing the lamellae to clump together, thereby reducing gas exchange. The clumping and excess mucus may also protect parasites from the effects of medicines. In cases of hyperplasia, is important to test the water, do partial water changes if indicated, and add fresh carbon. Irritation may also be caused by parasites, you could treat with praziquantel for flukes and/or malachite green/formalin for other external parasites.

Once the lamellae clump together this will affect gaseous exchange and respiration negatively.

Pictures (microscopic) of Gill filaments stuck together gills gills

Here the secondary lamellae (SL), the parts of the gills that exchange oxygen and carbon dioxide, are clumped together
by mucus.

Frayed Gills If the gills are frayed in the ends, they have probably been 'burned' by chemicals like ammonia or by very low pH. Sometimes formalin or potassium permanganate can cause fraying. If you see fraying and no other symptoms, it is probably best to keep the water pristine and let the gills heal on their own.
Bleeding from Gills

If the fish has been recently netted or handled, this could cause bleeding. Also read about koi bleeding from gills here. Another likely cause is water quality or parasites (like flukes), but it could also be bacterial gill disease. Make sure the water is pristine, run the fish through a parasite treatment (flukes and other parasites), and feed medicated food (medi-gold preferably).

Gills clumped together and eroding. Yellow color on gills or hamburger-like gill consistency

If the gills are clumped together and eroded, often with yellow patches, this is likely bacterial gill disease. Typically this is caused by bacteria (e.g. Flavobacterium columnaris) after the gills have been damaged by water quality issues or parasites.

See further description here . A good picture here (in this article).

Treatment should include treating any parasites that caused tissue damage opening up the gills to bacterial infection. Look for other external symptoms of parasites and treat with formalin/malachite green. If flukes are present, treat with praziquantel. Salt in the water (0.3%) for salt tolerant fish or even 0.1% may help. The bacterial issues can be addressed by antibacterial food, if the fish is eating, or antibacterial water medication (like Maracyn Plus). BGDX Powder (containing p-toluene sulfonyichioride) may also be a good medication here.

Pictures (microscopic) of gills from a
Celebes rainbowfish with bacterial infection and possible ammonia damage

Arrows are indicating areas with problem. The problem is manifested as black mass. Close-up of same gill. The blue box is around columnaris "haystacks"

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Stringy White Poop

Diagnosing fecal problems is not always easy. But for most fish, white stringy poop is a sign of a problem and something to take notice off . However, poop will be the color of the food fed so if you are feeding white, it is probably natural.

Causes and Treatment
Stress Stress can cause white poop. For a fish, stress is bad water quality or changes in water parameters. Measure for ammonia, nitrite, nitrate, and pH. Check the temperature. Aggressive tank mates may also lead to stress, as can illness and medication. Moving to a new tank or the addition of new tankmates can also cause stress.

The parasite hexamita, that can attack internal organs, very often causes white stringy poop. Feed the fish anti-parasite food that contains metrodonidazole.

Infection Bacterial infections can lead to stringy white poop. Before acting check for other signs of infection (redness, lethargy, lack of appetite).
Goldfish with long stringy white poop

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Floating and Sinking
Definition There are times when we can find our fish either sitting on the bottom incapable or hesitant to swim around, or other times the fish is floating at the top sometimes even flipped over. These problems of buoyancy are complex and often quite serious. GAB has a good article discussing buoyancy.
Causes and Treatments
Water Quality Floating or sinking can be caused by poor water quality. Many people think that high nitrate may cause floatiness in goldfish. So if you notice buoyancy problems, check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

If your fish lose control of their swimming, they may be constipated. Some fish that are constipated may bottom sit. Take care to check the nutritional requirements for your fish. Do not over feed. If you think constipation could explain the symptom, fast your fish for 3 days. You may try to feed a piece of pea after the fasting period.

Gassiness Many people believe that when fish get floaty, excess gas in their digestive system may be the cause. Fish could get gassy by swallowing too much air or by eating food that releases gasses in the digestive system. Read more about food and gas here. If the fish is floaty, avoid feeding foods that may cause gas (such as soy or wheat products). Floaty poop may be a further indication of gas in the GI tract.
Infection Illness can cause buoyancy problems. Internal infection can interfere with the fish's ability to regulate gas density in the air bladder and cause buoyancy problems. If there are signs of infection (e.g. lack of appetite, lethargy, redness, etc), feed medicated food, or if the fish is not eating add a broad spectrum antibiotic to the water. For larger fish, antibiotic injections are preferred. Bacterial infections are often secondary to wounds caused by parasites such as flukes, so if you have access to a microscope, doing a scrape and scope to look for parasites is recommended.
Air-Bladder malfunction The air-bladder (or swim bladder) is not a very well understood organ. It seems to be linked to sensory functions like hearing, and some fish use it to produce sound. For fish that have air bladders, the organ is also used to control buoyancy. The gasses inside the bladder (which are not air) are compressed or decompressed (probably through the tissue outside). See explanation in this article. It is possible that ailments (bacteria, parasites, virus) may cause buoyancy problems, either by affecting the small blood vessels that feed gasses into the swim bladder or because as the fish is too weak to use the muscles around the air bladder. Water quality (like high nitrite or nitrate) may also cause the fish to experience buoyancy problems. Internal cysts and tumors may also cause swim bladder malfunction by physically compressing the organ.
Genetic problems Some fish (like fancy goldfish) are bred to have short round bodies and this may cause the fish to have less room for vital organs such as the air bladder. As a result the fish may experience buoyancy problems. Some fish may also have deformed swim bladders.
Picture (x-ray) of goldfish with swimbladder displacement
This is an x-ray of a goldfish with a deformed swim bladder. The caudal lobe is over-inflated and displaced laterally. This goldfish has issues with buoyancy and swimming straight

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Losing Weight
Definition If your fish fails to thrive or is losing weight despite eating, you are looking at a couple of likely explanations.
Causes and Treatments
Internal Parasites If you are feeding regularly and the fish is still losing weight, you may be dealing with internal parasites such as nematodes, cestodes, or flagellates. Treat with Jungle Antiparasite Food which contains metronidazole, praziquantel, and levamisole. Praziquantel and levamisole are also effective de-wormers as bath treatments.

Hexamita (a flagellate, Spironucleus vortens) is a common internal parasite in some species but hard to diagnose. Weight loss, gill fluffing, and spitting out food are common signs. The best treatment for hexamita is metronidazole available in products such as Jungle Antiparasite Food or Metro-Med.

Fish Tuberculosis If treating for internal parasites does not help, there may be more serious issues like fish TB (more correctly called mycobacteriosis) or other viruses. If that is the case, the fish should be isolated. Mycobacteriosis is incurable.

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Losing color
Definition If the colours of your fish are less vibrant or if they turn pale, this may be a sign of problems.
Causes and Treatments
Water Quality

Loss of colour may be due to poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum.

Stress Stress is often a factor in fish changing colour. Stress can occur in response to stressors. Common stressors include: handling, changing water parameters, water changes, aggressive tankmates, changes in temperature or the wrong temperature, noise, vibration, sudden movements, or overstocking/crowding. Try to identify the source of the stress and alleviate it. Stress often leads to illness because it depresses immune response.
Bacterial Illness Some fish get pale when fighting an illness or an infection. Make sure you look for other signs of illness (e.g. redness, lethargy, loss of appetite, etc) and medicate appropriately with medicated food if the fish is eating or broad spectrum bath antibiotics if the fish is not still eating.
Costia Costia is a protozoan parasite, Ichthyobodo, that attacks the fish's skin. This attack often causes the fish to produce more slimecoat, and as a result the fish may look whiter or paler. Read here about Bugsy the black moor's costia to see good pictures of scopes of the parasite. If the whiteness is caused by costia, you should be able with closer inspection to see the slimecoat changes. The fish will often also flash and rub because of the irritation. Costia is best treated with a malachite green/formalin combination present in parasite medications like Rid Ich or Quick Cure.
Natural Colour Changes Some fish, like goldfish, will change colour over time. Goldfish may lose their orange, red, or black colour due to environmental changes or genetic makeup. You can read more about goldfish and colour changes here.
Neon Tetra Disease

Tetras and perhaps other cyprinids are susceptible to a sporozoan parasite named Pleistophora hyphessobryconis. The parasite attacks the intestines and eventually muscle tissue of the fish and will cause paleness. Neon tetras, for whom this disease is named, will lose the blue colour in their middle colour band.

There is no known treatment and it is recommended to euthanize infected fish to prevent spreading. However neon tetras often react to stress by paling, so be sure you have eliminated other causes before taking this step.

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Bent Spine
Definition If you look at your fish from above and you see a bent spine or an S curvature, there are several possible causes.
Causes and Treatments
Birth Defect

Bent spine is a common genetic birth defect and should be culled out. Fish with this condition should never be allowed to breed. Sometimes instead forming an "S" shape, birth defects result in a "U" shaped spine.


Malnutrition may cause spinal deformities so take care to check your fish's nutritional requirements and feed fish every day. Lack of Vitamin C is sometimes identified as a cause of scolosis, so take care to properly store your fish food and be sure it is not too old. A good rule of thumb is to toss out any foods that have been opened for 6 months. Fresh food may be a good source of vitamin C as well.
Injury Fish may become injured from fleeing (fight or flight reactions), jumping against things when spawning, aggression, during tank maintenance, etc. If this occurs, keep the water pristine and reduce the cause of the aggression or flight response. In severe cases fish can break their spine.
Electricity If your fish is outside in a pond, a bent spine can be caused by lightening strikes. Equipment leaking electricity in the tank can also result in a bent spine if the fish becomes grounded.

Myco-bacteriosis or
Fish Tuberculosis

A bent spine is one of the signs that the fish in infected with bacteria called Mycobacteria. Some people refer to this condition as "fish tuberculosis" or "fish TB." Mycobacteria (there are several kinds) are gram-positive bacteria that infect slowly and gradually cause more and more problems for the fish. Other symptoms may include emaciation, redness, lethargy and exophthalmia (pop-eye). If this infection spreads to the skeleton of the fish, the spine may bend. There is not currently any treatment for mycobacteriosis. Isolation or euthanasia are recommended to avoid spreading.

Many people believe that fish that are over crowded and lack nutrious food are far more susceptible to this infection than fish kept in good conditions. So it is a disease easier to prevent than cure. Read more about Mycobacteria here.

Mycobacteria can cause "granulomatous skin disease" or "fish handlers disease" in humans. It enters through breaks in the skin. If you suspect fish TB, avoid placing hands or arms with open sores in the aquarium. Cleaning hands with anti-bacterial gel hand sanitizers should be done as a precaution. People with a compromised immune system should avoid coming in contact with aquariums where fish TB is suspected.

Other causes Internal swellings, tumors, or not being able to expel eggs can also cause bent fish because the pressure or pain from the swelling/tumor causes them to favor one side.

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Definition If fish get bloated (swollen in the belly area), you may be dealing with constipation, gas, or retention of fluids.
Causes and Treatments

Constipation may happen if the fish is fed too much or fed the wrong foods. Fasting may help here. Review the food you are feeding and make sure you're feeding the correct type of food for your fish (e.g. omnivore, herbivore, carnivore). Eliminate foods that may cause gassiness (soy may cause gas in the gastrointestinal tract). Try adding more fresh foods to the diet. Feeding a pea sometimes helps. If peas don't help, you can add a small grain of Epsom salt inside the pea.

Fluids Check the gills of the fish. If they look pale, organ failure is likely. Organ failure can cause the accumulation of fluids in the viseral cavity (where the organs are). Also look for signs of onset of dropsy.
Eggs or egg bound Female fish that carry eggs may also get very round. In rarer cases if the eggs are not released or expelled, the fish may get "egg bound" or egg impacted. There may not be a cure for this. With larger fish (like large goldfish) one may be able to stimulate egg laying. If a fish is truely impacted, she will need surgery. There is some anecdotal evidence that feeding fish a grain of epsom salt may help.
Kidney problems or internal dysfunctions Bloating can be caused by serious internal problems. There may be an infection or a tumor, or the kidneys may not be working properly. There cases are hard to diagnose without help from a veterinarian. If the fish is lethargic and bloated, it may be worth trying antibiotics - in food if possible.
Bloated goldfish with cyst
Oranda goldfish with a cyst in her belly.

X-ray of the same fish. Read about her vet visits.
Dropsy See Dropsy

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Definition Dropsy is a symptom that the fish is in kidney failure and is not able to excrete enough fluid. This generally causes swelling of the belly that is usually accompanied by raised scales that make the fish look like a pine cone. It is difficult to save a fish once this happens, because the kidney is already compromised. Most cases are fatal. Check the gills. If the gills are pale, the fish is in organ failure and should be euthanized humanely.
Picture of Dropsy
Causes and Treatments
Organ failure

Check the gills of the fish. If they look pale, organ failure is likely. The likelihood of saving a fish with organ failure is very small

Other Causes

Diseases that can cause dropsy include bacterial, parasitic and viral infections. Toxins in the water or food can also cause kidney failure. The only thing we can do is to provide supportive care and treat as if there is a bacterial cause. If the fish is large enough, antibiotic injections would be the treatment of choice. For smaller fish who are still eating, feed an antibiotic food. If the fish is not eating and not large enough to inject, use a broad spectrum antibiotic bath. Avoid kanamycin and other aminoglycoside type antibiotics (e.g. amikacin), as they can be nephrotoxic. Many people recommend adding epsom salt (magnesium sulfate) to the water -- we do not believe this helps; however, for hardwater fish, adding 250 ppm calcium chloride may help reduce the energy needed for osmoregulation. Adding salt (sodium chloride) for salt tolerant fish, may also help support osmoregulation.

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