While female fish can fill up with eggs, and some get egg bound, I think this is something else - the bloating is further forward than where the ovaries are located. Since the fish still seems to have proper buoyancy, that leaves ascites, fluid build up. In marine fish, this is either caused by liver or kidney failure. Sorry, but just like with people, there is no treatment for this (that’s why people need transplants).
Pain perception in fish is not clear to me. I let the fish guide me…if the fish is still eating and can carry out basic life processes, I generally hold off euthanizing, just in case it might recover. I’ve had electric eels live for years with ascites.
