Not mine, but I have used it before:
"Popeye or exophthalmia is a symptom, not a disease in itself and it can have a large number of potential causes. Some of these causes are incurable while others can potentially be cured on a sporadic basis. The bulging or protruding eye, as the name 'popeye' implies, is symptomatic of this condition.
Potential causes include --- infrequent water changes which results in a buildup of dissolved waste products in the water, Ichthyosporidium -- a parasitic fungus, Ichthyophonus [NOT to be confused with Ichthyophthirius, the parasite that causes "ich"/whitespot disease], 'worm cataract disease' (a function of the invasion of parasitic trematodes or flukes), bacterial infection, parasite infestation by eye flukes, and internal metabolic disorders.
Since the causes of popeye are so varied, the treatment is difficult. Some success has been noted in treatment of Ichthyosporidium using one percent phenoxyethanol at about 50gm per gallon of water.
Treatment for bacterial disease would best be accomplished using tetracycline or teramycin added to aquarium water every other day or mixing food at the rate of 100 mg of antibiotic to 4 oz. of food. Feeding should continue 10 days.
Fish tuberculosis can also be responsible for pop-eye. Since fish tuberculosis is a bacterial disease caused by such as Mycobacterium, the same antibiotics can be used in the same manner for treatment if this is the suspected cause. Other effective treatments for bacterial exophthalia may include chlortetracycline, furazolidone, nifurpirinol, oxolinic acid, oxytetracycline or potentiated sulphonamide.
Unfortunately, these treatments are most effective when injected into the eye socket and less so when used as a bath.
If the fish has eye flukes, malachite green with formalin, metriphonate or copper are good, as are most of the commercial preps for flukes and external parasites.
Popeye is rarely fatal and not particularly infectious. Bear in mind that many of these treatments will negatively impact your biological filter so treatment in an isolation tank is best"
Pop-eye CAN go away on it's own However based on it's cause as the article says it may not, it's best to QT, observe and treat.
Has your tank had changes lately? Overstocked? If not I would suspect it's Bacterial and treat.
