Pop-eye is a condition resulting from injury, bacterial infection or poor water quality. Pop-eye can be fatal if left untreated in koi. As the name suggests, the condition results in the fish's eye appearing to pop-out of the socket. Distended eyes are a result of inflammation in the eye or around the surrounding tissue. Treatment of the condition is largely determined by accurate diagnosis of the cause. A hospital tank is recommended during treatment of non-water quality issues to reduce incidental damage to the koi or health of the pond ecosystem.
Closely inspect the fish's eye for damage. Abrasions from erratic swimming, rough surfaces in the aquarium or damage by other fish may cause swelling of the eye. If necessary, catch the fish with a large net and inspect the fish while it is at the water's surface, in the net. Avoid prolonged removal of the fish from the water for inspection; stress will aggravate the condition.
Identify how many fish in the pond or aquarium display signs of pop-eye. It is rare that more than one fish will be fighting bacterial infection. If many fish are displaying pop-eye, water quality is the likely culprit.
Determine if one or both eyes are displaying pop-eye. If both eyes are displaying pop-eye, the cause is likely bacterial infection.
Measure the water quality. Use the aquarium test kit to measure the ammonia, nitrite and nitrate levels in the aquarium water. Also inspect the behaviour of the fish to determine if they appear to be lacking oxygen. Fish stressed by poor water conditions are more susceptible to disease.
Perform water changes on the pond or aquarium. Replace 30 per cent of the water every four to five days until the water quality is restored.
Address the root cause of the water quality issue. Overfeeding, overstocking, inadequate filtration, inadequate oxygen exchange with the water surface, poor maintenance and toxin contamination all impact water quality. Your fish's environment will return to a poor state unless root cause conditions are addressed.
Move the fish to a hospital tank. Unless you are treating for water quality issues, move the fish to a smaller, more stable environment for treatment. Hospital tanks may be made from large clean tubs, new rubber dustbins or water troughs. Rinse the hospital tank to remove any contaminants. Fill the tank with 70% water from the aquarium or pond and 30% new dechlorinated water.
Treat the hospital tank with antibiotics. Maracyn I and Maracyn II are effective for treating pop-eye, but like all antibacterial agents, the chemicals will destroy the biological balance in the aquarium or pond. The beneficial bacteria in an aquarium or pond that break down ammonia and nitrite will be damaged by antibiotics. Treat the hospital tank according to directions for the full duration of time. Replace 20 per cent of the water in the hospital tank with newly-treated water on a daily basis during treatment.
Ease the fish's stress by adding 1 tsp of rock salt to the hospital tank or aquarium for every 5 gallons of water. A slightly higher salinity will not harm your fish, but will aid your fish's breathing by improving the oxygen exchange between the water and the fish's gills.