Im fairly sure a water change wont do anything.
That is incorrect
A water change can help to releive conditions that may have caused the popeye in the first place.
It is normally caused by environmental conditions like poor water quality, high DOC’s, high nitrates, improper catching like when using a net, fighting with other fish, or by having hit something hard in the tank like rocks, heaters, or any other stationary object. It can also be caused by bacteria, parasites, virus or a growth behind the eye.
In minor cases of Popeye the eye will usually return to normal without blindness. If more serious the eye may appear colorless, grey or have that clouded look, and could result in blindness in the eye. If both eyes are affected and are severe the eyes may burst or disappear in which case the fish may not be able to recover and may die.
Treatment: Overwhelmingly it appears that the first step to take is to do a large water change. At least 30%. This can be done in the main tank with fresh saltwater that has been vigorously airiated and mixed for at the very least a 24 hour period, using 2 or 3 day aged water would be better. Raw saltwater,freshly mixed and not aged and aerated is stressful to fish and highly irritating to sensitive gill tissues. Why the water change? This will help remove DOC’s and Nitrates and improve the condition of the overall system, providing a cleaner and healthier environment for the fish to recuperate in, quite possibly removing that condition which caused the Popeye in the first place. In most minor cases the injury will heal itself, however it also depends on what caused the condition in the first place.
THomas