Fin rot in koi is caused by bacteria that destroy the fin tissue. This condition is typically associated with stress, caused by overcrowding, aggression, poor diet and poor water quality. These conditions weaken the koi's immune system and predispose the fish to fin rot. Although fin rot is not a fatal disease, it indicates that conditions in the pond are below standard. Fortunately, fins treated in time grow back without deformity or scarring. The aquarist must also correct all conditions that contributed to fin rot, while treating the fin problem itself. Poor water quality must be corrected immediately to assist the koi with recovery.
Use an ammonia test kit to measure the ammonia level in the pond water. Ammonia is highly toxic and there should be no trace of it in the test sample.
Use a nitrite test kit to measure the nitrite level in the pond water. Nitrite is only slightly less toxic than ammonia, and no more than 0.25 ppm should be present in the water sample.
Check that your biological filter is functional, if there are traces of ammonia or too much nitrite in the water. The pump that supplies pond water to the filter box might not be working, or the biological filter material may be smothered with slime and detritus.
Open the filter box and remove the biological filter material and rinse it in a plastic container filled with pond water. Do not rinse in tap water or you will destroy the beneficial bacteria that are required to convert the ammonia into less-toxic products.
Remove the mechanical filter material and rinse well under running tap water, or replace with new material if it is too soiled.
Remove the original activated carbon and replace with new.
Replace the filter box lid and start or repair the pump, if it was the problem.
Remove some of the koi to a separate pond, if the original pond is overstocked.
Examine the koi closely for fins with torn and ragged edges. These areas may have a red appearance and may be inflamed. Fungus might also be present.
Ask your veterinarian to trim away dying fin tissue in the case of advanced fin rot. The veterinarian will sedate the fish in a separate container and use sterile scissors to trim away the dead tissue. He will then treat the freshly trimmed edges with an antiseptic preparation.
Treat the entire pond with a commercial fin rot remedy, if most or all of the koi are suffering from this condition. Use the prescribed dosage of liquid medication and remove the activated carbon before treatment, as this product will absorb the medication and reduce its effectiveness. Keep your pond well oxygenated during the treatment period. Return the carbon to the filter after the treatment period has ended.
Remove koi to a separate plastic container for treatment, if only one or two fish have the condition.
Feed the fish the correct amount of a high-quality commercial koi food in the future. Nutritionally balanced food will allow the koi's immune system to function optimally and help prevent fin rot.