Well it sounds like bloat to me. How long has it been swollen... if they grossly over eat they can stay swelled up for some time while they digest their food... if it has been the same over a period of a few days you might have something to worry about. Bloat is usually caused by 2 main sources; feeding cold or frozen food, and abdominal dropsy.
You should never feed cold or frozen food to fish because they are cold blooded and they can only be as warm as the water they are swimming. So frozen food usually stays frozen for long periods of time. I would rase the temp up to 82* F and start to feed a vitamin rich diet, in much smaller portions so they are easy to digest.
If it is dropsy then you should treat like this: 1 g of tetracycline to every 100 lires of water for 4 days.
Add 1 tbs of kosher salt to every 5 gallons of water, and raise the temp to 80-82* F, keep this treatment up for a few weeks or until the fish is back to normal and healthy for a few days at least
Hope that helps, let us know how it goes. Others might have some differant ideas and treatments to suggest as well.
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Adam
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