Goldfish and Aquarium Board Articles
What Fish Gills Look Like
What should healthy gills look like?
They should be bright red, crisp and well defined.
Photo by verry sweet
Here are some other pics that may help.
The gills in the initial pictures here are not happy. The site has some nice info on anatomy and good microscopic pics.
The AKCA Koi Health Advisor Anatomy module (pdf) also has drawings and information about how the gills work.
Here's gill info from Frank Prince Iles on one of Doc Johnson's sites. Nice info on how gills work along, microscopic pics of what hyperplasia looks like, and info about the importance of catching gill problems early.
Diseases or Disorders of the Gill from KoiCrisis - nice illustrations of various gill issues.
Ulcerative Bacterial Dermatitis of Koi (Cyprinus carpio) and Ornamental Goldfish (Carassius auratus auratus) - Nice picture of healthy and damaged gill filaments.
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