Eye problems with betta fish

Question
My sister's betta fish has been acting strange for a couple of weeks.  She
thought he was sick and today we found out she was right when we noticed his
eye was bulging.  He also isn't using his fin on that side.  That side just seems to
be swollen.  His other side and eye looks fine.  He still eats but not as fast or as
much.  Did he scrape it on something or does he have an infection?  Is there
anything we can give him?   Please help.

Answer
It is probably pop-eye. I'm not sure about the fin but the eye is probably a case of pop-eye.
Do 40 percent water changes every few days and constantly monitor the nitrite and ammonia of the water. They should both be 0.0, even 0.1 is poisonous. If the fish has suitable, clean water, the pop-eye should subside given time.
I hope he gets better,
Kathryn