I am afraid that I must disagree with the above comments. Popeye is symptom, as opposed to a disease, caused by a build up of fluid behind the eye. The pressure forces the eye partially out of its socket, hence the eye looks like it is "popping out". It can effect one or both eyes. Trauma can be responsible, but often it is an infection that can either be of bacterial, viral, or fungal origin. There is no way to tell. Poor water quality can be responsible. Supersaturation of the water with oxygen can be responsible but this cause is uncommon. Thus, the best course of action is to treat the fish with broad-spectrum (Maracyn I and II) antibiotics in a QT tank. If left untreated, the pressure can build up enough to actually cause loss of the eye.