To be able to help out, we are going to need to know some more information. First of all, what are your water stats, ammonia, nitrite, nitrate, and pH? How big is the tank, and what other fish are in there? How long has the tank been set up for, and also what is your tank maintence routine?
Secondly, can you describe the lump a little more? Does the lump seem to be inside the fish, where you are noticing swelling in his abdomen, or is it on his skin, something external? If it is external, can you describe the color, size, and shape? If it is internal, can you see his scales sticking out like a pine cone?
Look at hish gils, do they seem to be red and inflamed? Is there extra mucous? Has he been scratching on objects in the tank? Are the edges smoothe or rough? What color are they?
