MY goldfish book says POP EYE:
Fish's eyes begin to protrude in a very abnormal way (not contagious)
Cause: poor water conditions
Treatment: Improve water conditions
I would try a water change of about 25% and use an aquasafe water conditioner if using tap water to protect your fish from minerals and chlorine harmful to your fish. It is relatively inexpensive and you only apply 1 tsp per 10 gal
of water. I don't know anything else about the problem and the book only gives a brief description but you may be able to find more info if you research the disease on the net.
Hope your fish gets better!