If you have over 3 watts per gallon of light over you tank, the plants start growing so fast they use up the carbon dioxide (CO₂) dissolved in the water. CO₂ becomes the limiting factor in plant growth. You can overcome that by either adding Fluorish Excel or by adding CO₂ to the tank. Rather than spending lots of money buying a pressurized tank, controllers, etc, this is the cheap way to add CO₂.

To make the CO₂ generator, you need an empty 2 liter pop bottle, some silicone, and 2-3 foot of airline tubing. I used a nail heated up on the stove to burn a hole in the cap the size of the airline tubing. then stuck the tubing into the hole and seal both sides with silicone, leaving the long end coming out of the top of the cap.

Give it 24 hours to dry.

I use gelatine to set my CO₂ generating mixture because it makes it last a month instead of only two weeks, because the yeast can't get to all the sugar at once.

To make the CO₂ generating solution for a 2 liter pop bottle you'll need:
2 cups sugar
1/2 tsp yeast
1/2 tsp baking soda (Optional)

for the gelatine you can use either
2 packs unflavored gelatine or
2 6oz packs of jello (with this I'd dissolve the gelatine in the amount of water from the package instructions, then add enough water to fill the two liter as below)

Make sure the bottle and your other equipment (e.g. funnel) are clean.

• Dissolve the unflavored gelatine in 2 cups of cold water, then bring to a boil stirring until gelatine is dissolved.
• Then pour into the 2 liter pop bottle and fill to about half way with cold water.
• Add the sugar and baking soda and shake untill it's dissolved well.
• Fill with cold water until it's about an inch below where the top of the bottle starts to curve and shake again.
• Refrigerate until the gelatine sets.

Add warm water up to the point the bottle starts to curve, then add the yeast and a tablesppon of sugar. Then swirl around to mix. (or you could do this in a cup and stir, then pour into the bottle). The solution should be at least 3 inches below the top of the bottle.

It's ready to cap and hook to the tank. As the yeast wake up and start munching on the sugar, they will start producing CO₂.

Here's a picture of my 3 liter bottle shortly after it was first set up (3/5/06). You can see the bottom is still gelled. The yeast works its way down over time.

and here it is on 3/17/06

Some people add a one way valve to make sure it won't back syphon tank water into the bottle and others add a second bottle between the CO₂ generator and the tank (see the links below).

You can hook the airline hose to an airstone, but a lot of the CO₂ just bubbles out. I've used the top part of a pop bottle as an umbrella over the airstone (the reactor) to give the CO₂ more contact time with the water--Not very efficient... but it helps. If you have a powerhead in the tank, you can run the airline tubing into the powerhead and let the impellor chop up the bubbles. I built a reactor this weekend to get more of the CO₂ dissolved in the water. It's basically an old gravel vacuum hooked to a powerhead. In the pic below, it's nestled behind the return from my sump (the white pvc and black hose).

I used an old gravel vacuum I picked up at a garage sale here while back. First I drilled out the little ball that was there to prevent backflow, drilled a hole for the airline tubing, and used silicone sealant to attach rigid airline tubing inside the gravel vac. I let that dry overnite and then used an adaptor that came with the powerhead and some hose from the gravel vac to hook the vac and the power head together. I stuffed some medium sized filter media in the bottom of the vacuum tube, hooked up the airline from the CO₂ bottle.. and viola. Well dissoved CO₂!

References

• A nice thread on DIY CO₂. They say you can drill the hole a bit smaller than the airline tubing and you won't have to use silicone sealant. They also use the second bottle to collect gunk. These guys add water to the second bottle and bubble the gas thru it to collect the slimey stuff that often ends up in the tank.
• The jello thread where I got the idea for using gelatine to set the mixture.
• You can find out how much CO₂ is dissolved in your aquarium if you know your pH and KH.

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