Why does a fish loose fins?

Answer 1

Sounds like fin rot. Bacterial infection usually caused by poor water conditions. Check your parameters with emphasis on ammonia and nitrites which cause undo stress on your fish in turn lowering its immune system. However this can also be caused by improper pH and/or temprature, pH and or temperature swings, other fish picking at it (although other fish tend to pick at fins after a bacterial infection has set in) and any other variable that may make the fish weaker than normal. If you catch it early try stepping up your water changes (in a manner that doesn't further stress the fish), bumping your temperature 3-5 degrees, and using aquarium salt. Quarantine it if possible remembering that you don't want to drop it into an uncycled tank and risking an ammonia spike. If not caught early I would try Mardel's Maracyn treatment in which case you really would want to do a hospital tank since it would screw with your nitrogen cycle.