Pop Eye is a generic term for Exophthalmos, and refers to a condition that causes the of the fish to abnormally bulge from its socket. Popeye can be caused from bacterial diseases, abcesses and tumors, eye injuries, as well as over-aerated water. If left untreated, the eye may bulge so far from its socket that it pops out, resulting in the loss of the eye.
Treatment: First, eliminate the possibility of over-aeration. If you have been using highpowered power heads that shoot streams of air into the tank water, your fish may be victim of nitrogen supersaturation. The tiny air bubbles enter the fish's bloodstream and can accumualate behind the eye, causing it to protrude. Shut off all extra power heads temporarily allow only the air pump to be the only source of air, but leave your main return on (of course).If after a few days there are no improvements then it is not nitrogen supersaturation and follow the treatment recommended for Bacterial Infection.
Hope this helps