"Fin Rot" is used to describe any of a number of infections, illnesses or conditions that cause a fish's fin(s) to appear frayed, torn or even missing.

Aggression. The most obvious cause of damaged fins is aggression by other tankmates. Most often, the tail is affected first, and long, clean slits appear between the fin rays. In the case of cichlids, other fins will usually be damaged quickly, and scales will begin to disappear as well. The biggest, most assertive fish that is undamaged is always the most likely suspect, but smaller "nippy" fish may also bear watching. Sometimes finding the culprit isn't easy, as fish behave differently when they know they are being watched. The aquarist may need to turn off the room lights one evening and sit quite still for 10 or 20 minutes before normal behavior resumes. Treatment: separate the offending party.

Chemistry. Fins sometimes seem to deteriorate all on their own in cases of extremely poor water quality (for example, very high nitrates and low pH) that are the result of overcrowding, overfeeding or inadequate water changing. Exposure to chlorinated tap water may also cause fin damage.Treatment: get caught up on water changes and reduce feeding size and number of fish if necessary. Properly dechlorinate water before adding to aquarium.

Infection. Quite a few parasites and bacteria can cause fins to decay. These may gain a foothold if the fin is already damaged from handling or aggression, or if the fish's disease resistance is lowered by stress. Excess mucous and/or red blotches at the wound's edge indicate the likelihood of infection. Unfortunately, since any one of several different pathogens might be involved, there is no one simple cure. Treatment: identify the pathogen (preferably by microscopic examination), then apply the appropriate antibiotic or antiparasitic drug.

Healing. After the cause of "fin rot" is addressed, fish can heal quite quickly, often regenerating almost all the lost tissue within a few weeks. Good nutrition, especially in terms of vitamins, is essential during the healing process.