It could be 1 of 2 things 1- swim bladder or 2- a bad case of dropsy
1- a congenitally deformed swim bladder,cancer or tuberculosis in organs adjacent to the swim bladder,constipation,poor nutrition,chilling,serious parasitic infestations,serious bacterial infections if you suspect swim bladder in the fish you should first check and treat for other diseases. If you have eliminated other causes, make sure that you are giving right food and see if the fish is passing feces normally. If any doubt feed live food as roughage
2-Dropsy normally means fliud in the abdomen,but in fish keeping it includes all diseases which make tha abdomen swell abnormally. These diseases often can't be distinguished from each other without a post-mortem. A swollen abdomen that developes slowly is generally due to cancer internal organs or the presence of large parasites(the effect of thier size alone, or because they cause liver or kidney damage or peritonitis). In such cases it won't spreadto your other fish so long as you remove it.Acute dropsy is a sudden swelling of the scales to stand out, the most common problem is bacterial septicemia--veterinary treatment will be necessary.
This is about as much info that i know that might help you if you decide to keep it.