Its really hard to dianose fish without actually seeing the fish in person. It is hard to say what this might be? You have to be a detective, and thru the process of elimination somtimes you can figure it out. Could it be an injury from rocks? Is a parasite? Is someone else beating him up, there are so many possibilities. Is it poor water conditions, wrong foods, or tankmates.
I am very hesitant to medicate fish unless I know exactly what the issue is, treating fish with the wrong meds can be worse then not treating at all.
I wish I could be more helpful