Koi are beautiful, expensive fish for large tanks and outdoor ponds. They live many years when healthy. Fungus invades fish during stress and injury and affects the skin, gills and mouth areas. Fungus growth is caused by Saprolegnia fungus and looks like cotton or fur on fish skin and gills. It occurs when koi experience injury, disease and environmental stresses such as poor water quality and extremes in water temperatures. You should regularly examine your koi for signs of fungus, especially during stressful conditions, and treat them immediately.
Set up a hospital or isolation tank with optimal water temperature and aquarium salt. Be careful to use the correct ratio of salt to water -- 1 tbsp of aquarium salt per 5 gallons of aquarium water, because too much salt will hurt or kill fish instead of clearing up the fungus.
Remove any fish observed with fungus from the community tank or pond immediately and place them in the hospital tank for 10 days, changing half the water every other day. Be careful to keep the water temperature constant and avoid extreme temperature fluctuations. Be careful to keep the proper levels of salt to water during water exchanges.
Replace half the water in the community tank or pond and treat it with ammonia remover to clear and control high levels of ammonia and balance the oxygen levels in the water. Vacuum gravel and remove as much pond debris as possible.
Replace the filter in the community tank or pond with a fresh new filter when the affected fish are removed. Add a new aeration bubbler to introduce more oxygen into the water. Observe the remaining fish closely for signs of fungus.
Reduce the numbers of fish in the community tank and test the water. Keeping the water clean and oxygenated greatly reduces stress and disease-causing conditions.
Return the isolated fish to the community tank only after all fungus has cleared and the water environment is stabilised and stress conditions are eliminated.
Maintain a functioning and fish-ready hospital tank at all times so you can remove and treat fish at the first signs of fungus.
Continue consistent good fish keeping and water maintenance practices to avoid stressing the fish again. Regular water exchanges, proper filtration maintenance and close observance of the fish keep fish healthy.