Originally Posted by FordMan
Usually when a fish body bloats it's some kind of internal bacterial infection you can try antibiotics but most likely it's too late. Epson salt will make the problem worse you want the fish to excrete fluid not retain it which adding disolved solids to freshwater tends to do.
I'd have to disagree. Bettas are pretty prone to constipation, so I would suspect this before internal infection. Adding epsom salt actually do cause the fish to excrete fluids because of osmosis. Because the fish's cells can not take the salt in, to achieve equilibrium they let water escape. If you want to read more look up "osmosis in a selctively permeable membrane".
Before you go adding anything to the water though, I'd try the fast followed by feeding daphnia. It's just easier on the fish. We also need to find out the cause. What size tank is it, what and how much do you feed, what are the water parameters (ammonia, nitrite, and nitrate), and how long have you had the fish?