Popeye (exophthalmia) is a bacterial infection that is usually attributed to poor water conditions.
You will begin to notice your betas eyes are starting to look larger and they are beginning to bulge.
I have known quite a few people who have gotten rid of their beta fish when this happens thinking they are sick beyond recovery. In fact, Popeye is quite curable.
The first step in treatment is to clean your beta fish tank or bowel. You should also add some
"Maracyn 2" by Mardel. Maracyn-2 is a broad-spectrum antibiotic (minocycline) tablet used for
Popeye, to your beta fish tank.
With proper treatment the eyes should return to normal within 2 weeks or so.
Popeye was likely brought on at least in part due to water conditions. In order to endure your fish remain healthy you should change the water in the tank every few days or twice a week.