Fin-rot is a common aquarium disease that is usually caused by a bacterial pathogen such as Aeromonas, myxobacteria or Pseudomonas bacteria.
The symptoms of fin-rot include split, ragged, or worn fins, often with white patches or edges. Fin-rot may be confused with fin damage caused by fin nipping by other fish, but fin nipping often results in a 'clean' looking fin with missing parts, rather than an obviously infected area.
The bacterial pathogens responsible for fin-rot are always present in the water but the problem will only occur if the fish is already in bad condition due to other factors.
Common causes of fin-rot include stress caused by bad water conditions, overstocking, damage caused by fighting or fin-nipping, poor nutrition, or recent transporting. All these factors will weaken the fishes immune system to the point where it is no longer able to fend of attack by the fin-rot causing bacteria.
If left untreated, fin-rot can continually degenerate the fins and eventually attack the fish's body, resulting in death. The first course of action should be to remove any stress-causing factors and make sure that water conditions are at their best. Following this there are a number of antibacterial treatments that can be used on the whole aquarium. For some fish, such as many livebearers, the addition of aquarium salt can help treatment and prevention of fin-rot.
It should be noted that because fin-rot is almost always caused by bad aquarium conditions, it should be treated as a sign that something is wrong with the overall aquarium environment.