Your fish appears to be suffering from an ailment called Dropsy complicated by another disorder called swim bladder disorder.
The best way to treat this is by changing 50% of the water in the fish's tank, add 1/8 teaspoon of epsom salts per every 5 gallons of water and begin to treat the fish with a medicine called Betta Fix which can be easily found at most pet stores.
It would also be helpful to feed the fish 1 pea (without the husk) every day so as to clear out the fish's swim bladder.
Good luck!
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YOUR TROPICAL FISH RESOURCE
OVER 20 YEARS EXPERIENCE W/TROPICAL FISH---BREEDING, MAINTAINING, CURING