Most gill rot diseases are caused by fungus (branchiomyces, dimigrans, branchimyces sanguinis) with some caused by microscopic parasites such as Gyrodactylidae. A Koi may be inactive staying around the waterfall area gasping for air one day and dead the next without any outward signs indicating the cause of death. If a gill plate is opened, a fungus might be found that looks like a hand saw blade and the color changed from pink to gray.

If the disease is diagnosed early enough, the fish can be placed in a well-aerated large pond or dipped into a 2% salt solution. Methylene Blue or Malachite Green may also be used.

Also, an high ammonia level in the water can cause gill rot. It is a good idea to test water regularly to ensure good health in your Koi.

*Information on this page is an excerpt from "KOI" by Grant Fujita, available at the AKCA online bookstore.