Well its either a bacterial infection or tuberculosis. I would guess bacterial infection.
Treat with recommended medication for bacterial infection in a quarantine tank. lfs
will have the treatment for this common disease.
If for some strange reason that it is tuberculosis.
here is what to look for
Tuberculosisis a mycobacteria. it is quite contagious to that are of the same species as the infected fish and can be spread to tankmates of other species as well. Care should be taken if you suspect tuberculosis, since there is a slight chance of human infection from a tubercular fish. Rubber gloves should be worn whenever you place your hands in the aquarium or work with a potentially infected utensils.
Symptoms: Pale color, bulging eyes, rapid breathing at the surface, sluggish movement, hiding, swelling of the abdomen (giving a red line appearance), no appetite and pinched belly or a wasting away of the fish are possible symptoms of tuberculosis.
Tuberculosis is a very difficult disease to cure almost impossible. since it is a slow growing bacterial disease, the best medication is a broad spectrum gram negative antibiotic such as nitrofuran (furan) or kanamycin sulfate. If you have an ultraviolet light make sure to use to prevent spreading of the disease. Since this is a very difficult disease to cure almost impossible, it may be best to sacrifice the fish for the benefit of its tankmates, IF A PAINLESS INSTANT DEATH IS REQUIRED. THIS IS A LAST RESORT METHOD SO THE FISH DOES NOT SUFFER.
Like I said it probably is just a simple bacterial infection, but I give you this information because you never know. The simptoms are like many other diseases so it is very difficult to diagnose and is associated with poor water quality and poor nutrition.
Hope this helps