My precious betta has one inflamed gill. he isnt eating and spits out his food!!

hes in a 2 gal bowl!! He has a african dwarf frog with him!! I use water conditioner!! I changed his water 100% yesterday!! I change his water 100% every week!! He has a plant and a filter!! HOW DO I CURE HIM?!?! btw i just saved him from a pet store i think that was the cause.