Need tank size in gallons or litres, water stats in ammonia,nitrite,nitrate,and ph, how many fish and which type.
What do you feed the fish, as a bloated belly can be constipation through a poor diet, bacterial infection, to internal parasites, what does it look like when the fish goes to the toilet.
For now i would try some shelled peas, cook for a few minutes let cool down, pop out of shell, mush between fingers and add to the tank.