The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
I would say fin rot. I think that ich would be spreading to others and across the body with in a day or two. Did you test the water? Do a pwc? Sometimes that helps a lot. He could have been nipped and rot grew on the damaged part. Have you put anything in the tank to treat it?
Congrats. IMO, you should keep with the treatment for the whole time even if he is better. If I remember correctly Melafix is every day for 7 days, then do a pwc and put the carbon back in the filter.