Depends on what caused it. If it is scrapping around and causing a ruckus, then water quality may have been the culprit. If it was recently ill with some kind of external parasite, then water quality may have been the culprit.
Are you saying it has scale rot, like on its flanks and such, or the tips and leading edges of the fins are rotting? Either way, water quality could be the.....
Do some water changes, make them a little larger and more frequent than usual. The potential is there for other fish to get the same ailment if they were to duplicate the same circumstances as the affected fish. Medicate the tank with Melafix and Pimafix. Melaluca extract is awesome and that fish will be back in no time. Watch for improvement over a few days and quarantine if necessary. Fin rot is usually curable with little to no lasting effects on the fish.