in order to receive the best advice, it would be helpful if you could give us all the information you can about the tank. this includes: tank size, filtration, heating, stock, feeding schedule, water change schedule, ph, ammonia, nitrate, phosphate, the physical appearance of the fish, and anything else that can possibly be related to the health of the fish (aka the chemistry of the water). get together all the information possible and post it here and keep us updated the best you can and we'll try to get tp the bottom of any issues you're facing.
best of luck