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Fin Rot is a very common disease. It is almost always caused by poor water conditions. It is contagious, but most fish are robust enough to resist it given good water conditions. The primary cause is bad water conditions. The stress induced by poor water quality, or other factors like aggression, weakens the fish's immune system to the point where it can no longer fend off attacking bacteria. The fins will begin to shrink, appearing ragged or "bitten." Sometimes actual bites from other fish open up the fins to fin rot.

Diagnosis: The fin perimeter will begin to shrink in either an uneven, ragged way or outward in a circle, like a fire consuming paper. The edges where it is shrinking will turn cloudy and white. If you start to see any bit of white of the edges of the fins, go dig out the water tests.

Cause: Fin rot is caused by a normal bacteria in the water. It's usually harmless, but can begin to grow on the fish if it overwhelms the immune system, much like fungus on humans. Times of stress will reduce the fish's immune system to the point where it can't cope.

Cure: Fin rot can sometimes clear itself up if water conditions improve. Removal of decaying plants, three 20% water changes, and rinsing the filter cartridge improved the water to the point where the affected should recover on their own, and re-grow all their finnage. Otherwise, fin rot is extremely easy to treat once the stressor has been removed. Treatment can be as simple as a strong salt bath. For fish like livebearers that like salty water can go up to a tablespoon per gallon for a period of time. Average fish should be in about 3 teaspoons per gallon. Sensitive fish like catfish and loaches should only go up to about 2 teaspoons per gallon. Watch them for signs of lethargy, and remove in about 30 minutes. For more stubborn cases, nearly all antibiotics will work, namely melafix, maracide, or any other anti-biotic that says it's good for fin rot. If in doubt, call us.

It's that simple. Severe cases may require fin trimming and stitching. If the fin rot is allowed to reach the origin of the fin, or penducle, it may be deadly to the fish. However, if your fish are allowed to deteriorate this far, I feel you have bigger problems than fin rot.

http://www.petfish.net/fin_rot.htm