It is dropsy, a common disease amongst freshwater fish. I recently had a Wolfish that had its abdomen area swollen to the point where he was stuck flowing at the surface of the water (like a submarine filled with air remaining at the surface). I tried a recommend remedy, but my Wolfish could not swim properly, hunt, and has not eating for weeks. It was suffering and I had no choice but to put it down (freezing). But maybe there is still hope for you; here is the remedy:
"Treatment:
Dropsy is not very contagious; however, if a fish is diagnosed with dropsy, it is important to not remove it from the aquarium because the infected fish will contaminate the separate water and end up dying. If there are multiple fish, treat the afflicted fish in a specially established "sick tank" (Quarantine). Dropsy can spread from the ill fish, possibly causing stress among the other fish in the tank community. This extra stress may make the others vulnerable to dropsy or other forms of disease.
Treatment may consist of antibiotics targeting the causative agent. They work best in the very early stages of dropsy. A more hands-on approach is to raise the aquarium's temperature a few degrees - slightly higher than usual. Adding Epsom salts (Magnesium sulfate) to the water (at a rate of 20 mg/L) helps to encourage the fish to expel unnecessary damaging fluids." (Fish dropsy, wikipedia.org)