How do you treat popeye in betta fish?

Popeye is caused by an infection. Infections in turn are caused by poor (Unsanitary) water conditions. There is no cure for popeye. You can try a few water changes and improve the fishes chances of fighting the infection but prevention is always better than cure. If you read and stick to the rules that follow, you will have better success. The basic rules of fishkeeping are :- 1 inch of fish needs at least 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every week. Provided you stick to the above rules, keep the fish at the correct temperature for their species, and feed them properly your fish stand a good chance of survival. Fail in any and I can guarantee that your fish will be constantly getting sick.