DISEASE: POPEYE
GENERAL INFO: If you always keep your betta’s water very clean, he will probably never get Popeye. Popeye is a bacterial infection caused by poor water condition (in other words filthy water because you were too darn lazy to get off the couch and attend to your betta!!!). On occasion, it happens to conscientious breeders, too
)..
SYMPTOMS: One or both of Mr. Betta’s eyes start bulging out. In about 2 to 7 days the eye might look so grotesque you will be afraid to look at your betta. Casimodo on a bad day will look more attractive then your betta at that point!! Please do not destroy your betta! Bettas often have full recovery from it and look normal again, as if nothing had happened. During outbreak, betta may be less active, may stop eating.
TREATMENT: popeye is not fatal and often betta will fully recover. On occasion he may lose an eye. But if you catch it right away, he should be fine. Immediately do a full water change. Keep his water very clean, changing it every third day. After putting him in clean water, add the antibiotic Ampicilex (Aquatronics) to his water. This medication comes in capsules. A full capsule treats 10 gal of water. So for a 1/2 gallon of water, open the capsule and take the right proportion of powder and sprinkle on jar water. You may steer gently with a disposable plastic spoon. This is a white powder and will not affect the color of the water. Do not overmedicate! Once Betta’s eyes are back to normal, stop the medication. And keep his water clean from now on darn it!!
Source:
http://www.bettatalk.com/betta_diseases.htm