Cancer?

I have a 55 gal tank with 4 young red bellies. I got them dime size in mid-February and they have done fine. The largest is now about 3 1/2 inches long but I think that he is not long for this world. He is still active and eating well but both eyes a bulging almost out of his head like a bubble-eyed gold fish and his belly looks about 10 months pregnant. The water conditions are good with ammonia and nitrates barely registering at all. No new fish and no changes in tank. I have been feeding mostly shrimp but with occasional frozen brine shrimp or krill and a few flakes now and then. Any ideas? How do your give a piranha an enema?