The previous poster is right, it is Dropsy. It is caused by a bacterial infection. The bad part of dropsy is it is caused by a build up of fluids internally because of organ failure.
You can try to treat it with antibiotics, however most fish do not recover.
Since there is an infection, is it possible to isolate the fish to protect other fish. Even though it isn't very contagious, the problem is when the fish dies, others tend to nibble on its body, which can lead to further problems. Also, what are your water stats (ammonia, nitrite, nitrate, and ph)?