Oscar Fish Has Swollen Stomach
This is a problem that seems to crop up more often than not. Because a bloated, swollen abdomen is a symptom, not a disease, it can be extremely difficult to treat.
Unfortunately, in many cases, Oscars that develop severely swollen abdomens do not survive. By the time the symptoms have shown themselves, it is too late for the fish. There are various reasons why a fish will develop these symptoms. In some cases, there is a genetic problem with the fish. The fish may well be okay for the first few years, then it starts developing problems in the abdominal area. There wouldn't be any cure for this condition.
Fish can develop tumours which will cause swelling in the abdomen. Unfortunately, the only treatment for this is surgery which would probably be futile as the fish would probably not survive.
Internal bacteria or viral infections in fish can often result in them having a swollen abdomen/body. Quite often the eyes bulge and the scales are raised. This is sometimes referred to as "dropsy" This is an extremely serious condition. Remember that dropsy isn't an actual disease, it is actually a symptom of something more serious that is wrong with the fish. For this reason, it is very difficult to treat. If caught early, antibiotics might help. However, off-the-shelf may medication not work, you may have to seek professional advice and have injections administered. Again, the chances of this resulting in a cure are quite slim.
If your fish develops a balloon shaped abdomen that is soft and not hard like a tumour, this could possibly be internal organ disease which can result in fluid buildup in the abdominal cavity. There is no cure for this.
Occasionally, a severe blockage in the intestine, or constipation can cause a fish's abdomen to swell. Constipation can be very serious, often fatal if the intestine is totally blocked. If you think your fish is not passing any waste, then it may be worth treating it for constipation. Firstly, change its diet, do some research and find out what your fish should be eating. Have you been feeding it the wrong kind of food? If a change in diet doesn't make any difference, you could try soaking its food in medication. Some recognised products include Castor oil, glycerol, and medicinal paraffin oil. Try to use foods that so cop these medications, flake food works well as it absorbs the medication. I promise no guarantees that these treatments will work and cure the constipation.
If you notice your Oscar has developed a swollen stomach in a short space of time, it just may be that he has eaten too much. Oscars are very gluttonous and they will eat as much as they possibly can which can result in a swollen abdomen. If the fish is showing no other signs of ill-health such as refusing food, cease feeding and see if the swollen stomach reduces in size. If it does, reduce the amount of food you give the fish and keep observing it to see if it happens again.
I'll be blatantly honest with you, if your Oscar develops a severely bloated abdomen and stops eating, the chances are it will die. In many cases, it can take days, sometimes weeks for this to happen. In cases like this, the kindest thing you can do for the fish is to humanely kill it using either clove oil, or an overdose of anaesthetic.