Is the bulging in the stomach uneven? It can actually be dropsy or swim bladder disease
Dropsy
Symptoms: Bloating of the body, protruding scales.
Treatment
Feed Antibiotic food (Tetra, Hagen)
Swim-Bladder Disease
Swim bladder problems usually indicate another problem listed here. If you suspect swim-bladder problems in a fish, first check and treat it for other diseases as listed below:
*Congenitally deformed bladder.
*Cancer or tuberculosis in organs adjacent to the swim bladder.
*Constipation.
*Poor nutrition.
*Chilling or rapid fluctuations in temperature.
*Serious parasitic infestation.
*Serious bacterial infestation.
If you have eliminated other causes, make sure you are feeding the right food and make sure the fish is not constipated. Give it live food for awhile to ensure it is getting enough roughage. Also, check the temperature for your fish's requirements and keep the temperature stable.
Test your water, do a 25% water change and try not to change the temperature as this would stress the fish and worsen its condition. Just like any other living thing, the fish would have a higher chance of recovering if you stimulate it by keeping the water in good condition along with proper treatment.