Pop-Eye A very obvious swelling and/or cloudiness of the eyes, or around the eye. This is most often caused by a couple of things, either poor water quality or physical damage or abrasion to the eye of which a secondary infections can take advantage of, causing the obvious signs. Also in if the fish isn't decompressed correctly, or stressed, an air bubble could develop/get into the inside the eye, forcing the eye to pop out.
In case where the injury is caused by physical banging into something, like a rock, which basically gives the fish a "black eye", in time the swelling will usually go away on its own. Otherwise with good water quality the condition usually disappears on it own. In some instance an infection may develop behind the eye, which may require removing with a serine. This can lead to the lost of the eye, so it's not something that should be done as a first resort.