If it looks like a pinecone all over, it is likely dropsy. This is actually a symptom, not a disease. It means the fish's internal organs are shutting down and there is no cure.
If it is just the belly area, it may be constipated. Try feeding it a shelled pea, as peas are a fishy laxative. If this doesn't work Google the dose rates of bath in Epsom Salts.