If your little swimmer is putting on some weight, you're probably feeding him too much - a problem that's easy to fix. But there are two other possible reasons your fish is looking chubby: abnormal fluid accumulation, or enlargement or distention of body organs. If your fish's scales seem to angle away from his body when viewed from above, then your fish is probably not getting fat - he's getting sick.
Abdominal fluid buildup, or "dropsy," occurs when a kidney tumor, cyst or infection upsets a fish's internal electrolyte and fluid balances. Bacteria (like mycobacterium or aeromonas), fungus (like ichthyophonus) and protozoa (like coccidia) can all cause kidney malfunction.
Another cause of dropsy is liver deterioration, which leads to lower levels of albumin, a blood protein. When albumin levels drop, fluid leaves the blood, pools in the abdomen, and causes abdominal distention. Any organism that destroys enough liver cells can cause this phenomenon. Diseases such as mycobacteriosis (tuberculosis), nocardiosis, coccidiosis and intestinal parasite migration can also cause liver damage.
Vibrio and hexamita infections cause gas and fluid to pool in the intestinal tract, bloating your fish. Occasionally, roundworm larvae infestation can cause enough irritation in the abdominal cavity to stimulate excess fluid production.
Tumors affecting any internal organ can grow large enough to cause visible abdominal enlargement. It's hard to distinguish tumors from fluid accumulation without an X-ray or a sample of fluid and cells.
You probably can't determine the cause of your fish's abdominal enlargement without your veterinarian's help. See the doctor right away to give your fish every chance for recovery.
Doing research on your own may be a last resort - many books offer excellent information. Just remember to act sooner rather than later, for your pet's sake.