How long have you had this fish? What other fish are in the tank?
Water conditions and/or nutrition are two of the main reasons why most hobbiests have problems to begin with.
I know it's tedious, but we need to know what your water quality is and what you are feeding the fish in order to help you. We need to know your exact pH, kH, ammonia, nitrite, nitrate, salinity, and temperature? Without knowing this, we cannot rule out this being the problem. Great water conditions to you may not be great conditions to me, and vice versa.