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Diseases of Koi Fish That Cause Red Fins & Tails

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Koi need a lot of space to be healthy. In crowded conditions, they may develop various diseases, including fin rot. Fin rot often starts with red markings on their tails and fins. This disease can kill koi quickly, and requires rapid intervention to fix it. You can cure a koi of this problem if you catch it early enough, but you may have to address any underlying issues to ensure recovery.

Fin Rot Symptoms

Fin rot often starts with red streaks appearing on the fins of a koi. The streaking will progress to a ragged edge to the fin. The fin may also appear fuzzy, depending on the exact organism causing the infection. As the infection progresses, it can eat a koi's fins all the way down to the fish's body. The infections that cause fin rot can spread to the koi's body and kill the fish within the week.


Fin rot is a nonspecific opportunistic infection. Nonspecific means that multiple organisms, including both bacteria and fungus can cause it. An opportunistic infection is caused by bacteria or fungi that live in most aquarium water, but can only cause infection if the koi already has an injury or is weakened by stress. For example, if the koi are in an overcrowded aquarium or get injured in a fight, they can develop fin rot.

Treatment and Prevention

Many pet shops sell topical drugs to treat fin rot. However, fin rot is not contagious, and antibiotics can disrupt an pond or aquarium's capacity for biological filtration. For best results, remove your koi to a separate treatment aquarium for the duration. Additionally, you will want to figure out what led to the fin rot in the first place, since this disease is rare in clean water. If you see fin rot, check your ammonia and nitrite levels, do a water change and make sure the pond or aquarium is not overcrowded.


A few other things can resemble fin rot. Some koi change coloration at up to 3 years of age, so the sudden appearance of red fins may not be fin rot in young fish. Additionally, not all damage to the fins means fin rot. Injuries from fighting or handling can cause clean splits along the fin rays. If you don't see reddish coloration and the splits are clean, the fish does not have fin rot. Damage like this will typically heal on their own as long as you provide clean water and regular feedings.