I did not include this malady in any category because just about anything can cause this condition. The eye or eyes of the fish become quite extended (hence pop eye) and could lead to blindness or loss of the eye.
The first step in treating pop eye is determining the cause. Look for other symptoms that may not be overly obvious and proceed from there. After the cause has been determined and, if possible, corrected, the only thing left is to wait for regression. Again depending on the cause, the only fear is blindness. If you are attached to your pet separate him and just pay more attention to feeding.