What medicine have you been using to treat this? Is this also one of the fish that has become very thin and is not eating (from your other post)?
Here is a little more info for you on pop eye. I am not the author of this
Exophthalmia (a.k.a. Pop-Eye)
Symptoms:
Fish with exophthalmia will have eyes that are abnormally enlarged and protrude from their sockets. In severe cases the eyes may actually burst from the socket, leaving the fish eyeless. Eye swelling is caused by a build up of fluids in or behind the eye.
Cause:
There can be any number of causes for this condition. Possible causes include poor water quality, bacterial infections, viral infections, eye flukes or other parasitic infestations, or piscine tuberculosis. Even though the fish may recover, the protruding eye may remain that way. This should not adversely affect that fish.
Treatment:
Since there are so many possible causes a broad-spectrum antibiotic is recommended, such as Tetracycline, but this treatment cannot obviously guarantee success. Be sure to keep water quality in good condition.