based on your description could be fin or tail rot, or, bacterial infection. Hard to say. without seeing the fish. are the fins clamped, is the fish breathing rapidly, is the fish scratching, eating, swimming normal. everything about the fish needs to be considered.
Treatment if it is tail and fin rot is for a bacterial desease, and follow up with treatment for fungus. fish should be isolated and if you use uv keep it on 24/7 until the event is cleared up.
