If it's just the one eye, it's most likely an injury. Popeye will more commonly affect both eyes. The easiest thing to do is several large water changes with well aged/aerated SW
. It will help reduce the DOC
feeding the problem. Usually within a few days the swelling will abate. Also be sure the fish is fed food soaked in a good marine vitamin containing [acronym="Highly Unsaturated Fatty Acid"]HUFA
[/acronym]'s. This will help the fish's natural immune system fight the problem. More often than not, meds are not needed.