Found this info on the net:
Bloating Definition If fish get bloated (swollen in the belly area), you may be dealing with constipation, gas, or retention of fluids. Causes and Treatments Constipation
Constipation may happen if the fish is fed too much or fed the wrong foods. Fasting may help here. Review the food you are feeding and make sure you're feeding the correct type of food for your fish (e.g. omnivore, herbivore, carnivore). Eliminate foods that may cause gassiness (soy may cause gas in the gastrointestinal tract). Try adding more fresh foods to the diet. Feeding a pea sometimes helps. If peas don't help, you can add a small grain of Epsom salt inside the pea. Fluids Check the gills of the fish. If they look pale, organ failure is likely. Organ failure can cause the accumulation of fluids in the viseral cavity (where the organs are). Also look for signs of onset of dropsy. Eggs or egg bound Female fish that carry eggs may also get very round. In rarer cases if the eggs are not released or expelled, the fish may get "egg bound" or egg impacted. There may not be a cure for this. With larger fish (like large goldfish) one may be able to stimulate egg laying. If a fish is truely impacted, she will need surgery. There is some anecdotal evidence that feeding fish a grain of epsom salt may help. Kidney problems or internal dysfunctions Bloating can be caused by serious internal problems. There may be an infection or a tumor, or the kidneys may not be working properly. There cases are hard to diagnose without help from a veterinarian. If the fish is lethargic and bloated, it may be worth trying antibiotics - in food if possible.
Bloated goldfish with cyst
Oranda goldfish with a cyst in her belly.
X-ray of the same fish.
Read about her vet visits. Dropsy See
Dropsy
Dropsy Definition Dropsy is a symptom that the fish is in kidney failure and is not able to excrete enough fluid. This generally causes swelling of the belly that is usually accompanied by raised scales that make the fish look like a pine cone. It is difficult to save a fish once this happens, because the kidney is already compromised. Most cases are fatal. Check the
gills. If the gills are pale, the fish is in organ failure and should be
euthanized humanely. Picture of Dropsy
Causes and Treatments Organ failure
Check
the gills of the fish. If they look pale, organ failure is likely. The likelihood of saving a fish with organ failure is very small Other Causes
Diseases that can cause dropsy include bacterial, parasitic and viral infections. Toxins in the water or food can also cause kidney failure. The only thing we can do is to provide supportive care and treat as if there is a bacterial cause. If the fish is large enough, antibiotic injections would be the treatment of choice. For smaller fish who are still eating, feed an antibiotic food. If the fish is not eating and not large enough to
inject, use a broad spectrum antibiotic bath. Avoid kanamycin and other aminoglycoside type antibiotics (e.g. amikacin), as they can be nephrotoxic. Many people recommend adding epsom salt (magnesium sulfate) to the water -- we do not believe this helps; however, for hardwater fish, adding 250 ppm calcium chloride may help reduce the energy needed for osmoregulation. Adding salt (sodium chloride) for salt tolerant fish, may also help support osmoregulation.
