One of the most aptly named fish illnesses, pop-eye fish disease or exophthalmus is a condition that causes the eyes to bulge out from the head of a fish. Although pop-eye is very easy to identify it can be difficult to treat properly. The reason for this is because pop-eye has many different possible causes. Here are a few guidelines that may help you find the proper treatment for pop-eye fish disease.
If more than one fish in the aquarium have pop-eye at the same time, then it is likely a water quality issue. Test water parameters and do daily 30% water changes for 4 or 5 days.
If the eye was injured by scraping it on a rock or by a fight with another fish then the best thing to do is to quarantine the fish in a hospital tank. The fish can be treated with Maracyn or Maracyn II to prevent or eliminate infection once in the hospital tank.
If both eyes are affected this is more likely a bacterial infection. Remove infected fish from the main tank and put them into a hospital tank. Treat the hospital tank with Maracyn or Maracyn II. If you cannot catch the fish to place it in a hospital tank then the main tank can be treated with Maracyn or Maracyn II. This is not ideal because it kills off beneficial bacteria in your main aquarium.
Some people have reported success with treating the water with aquarium salt (around 1 tablespoon per 5 gallons of water). You can use this treatment in combination with antibiotics and water changes.
If water changes, antibiotics or salt treatments do not seem to be working after 4 or 5 days then the cause may be viral in nature or ichthyosporidium, excess blood gases caused by nitrogen supersaturation (what divers get when they come up to the surface too quickly). If either of these are the case there is no real treatment available. You can just try to keep water quality up through water changes.