Please don't go poke your fishes eyes out with a needle
Pop eye is a condition, not a specific disease, and it can be the result of a number of different causes. Possible causes include internal bacterial infections by Exophthalmus and Corneybacteriosis, trauma to the eye, and poor water quality.
Environmental factors can include factors such as high levels of ammonia, nitrite, or nitrate, or otherwise poor water quality. It can also be caused by a carbon dioxide level in the water that is too high as a result of carbon dioxide enrichment of the water. If the carbon dioxide is released as bubbles the fish can swallow them, and this can cause pop eye.
Trauma to the eye can cause pop eye.
If only one eye is infected, it is usually caused by an injury.
Pop eye caused by bacterial infection or environmental issues usually affects both eyes.
If pop eye on both eyes is followed by a fungal infection, it is usually bacterial pop eye.
If pop eye on both eyes is not followed by a fungal infection, the pop eye is usually caused by environmental issues.
Never wait for a fungal infection before starting treatment. If your fish displays pop eye on both eyes, you should immediately take the steps against both bacterial and environmental pop eye.
Treating pop eye
Different types of pop eye require different treatments. Pop eye caused by trauma can often heal by itself if given optimal conditions. Move the fish with pop eye to a tank where it can be alone and won't get stressed by other fish. Make sure to keep perfect water quality in the tank.
Pop eye caused by environmental factors can often be dealt with in a similar fashion. It is very important to improve water quality and other environmental issues to avoid more fish getting sick. Fish will never get well if you don't improve the environment.
Bacterial pop eye can be hard to treat as it is an internal infection and internal infections are always harder to treat than external since it is harder to get the meds to the place where they are needed. Treat bacterial pop eye with a broad spectrum antibiotic.