Polycystic kidney disease
This is a common disease in goldfish that causes a slow progressive swelling of the belly over many months or years. In some cases, the fish may swell up so much that they look like they may burst. It is thought to be due to a developmental /genetic abnormality or a parasite that damages the kidneys. Large cysts develop in the kidney which enlarge and occupy a large part of the body cavity. Although it is easily confirmed on an ultrasound scan, there is no treatment for the disease.