Goldfish popeye is a generic term used for an internal bacterial infection that causes fluid to build up behind the eye of a sick goldfish, making the eye bulge or “pop out”. Goldfish popeye is often the result of goldfish dropsy, an internal bacterial infection or goldfish tuberculosis. Both dropsy and tuberculosis are difficult to treat. Trauma might also cause a goldfish’s eye to bulge or even fall out due to swelling of the blood vessels. (see goldfish eye diseases for more goldfish eye problems.)
At first the eye or eyes of a sick goldfish will bulge slightly. As the goldfish popeye infection worsens, the eyes will appear popped out on top of fleshy bags of fluid. (looking much like a telescope goldfish.) These severely protruding eyes can be pick off by aggressive tank mates.
Most water treatments will fail to reach the internal bacterial infection and many water treatments are also hazardous to the biological filter; although an aquarium salt bath can help lower the osmotic pressure of the sick goldfish and help ease the swelling behind the eye. An injection of Baytril is an effective cure but injecting a goldfish with antibiotics should be left to the experts. More available treatments include Maracyn Plus Biospheres AntiBacteria Therapy from Mardel or Kanamycin (Kanaplex 5gm). Kanamycin is less harmful to the goldfish than Maracyn.
Related: Treatment Options for Sick Goldfish