Your beta fish has a disease called Dropsy, which is a bacterial parasite which causes abdomen swelling. It is extremely deadly if left untreated, however it is treatable.
Purchase a medication called Maracyn 2. There is also a Maracyn 1, so make sure you purchase the 2, as they are totally different. The medicine is made by the Mardel company, and is sold commercially in most pet stores. Follow the directions on the label for treatment.
Do a 25% water change before and after medicating the fish, and remove all carbon from your filter before starting the treatment.
Hope this helps!
I am NOT, nor do I claim to be, a Veterinarian.
Always check with your vet for any symptoms your pet exhibits.
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