Most commonly it will be from an injury and/or possibley elevated DOC
in the water. Easiest and best method is a few large water changes with well aged/aerated SW
. Usually once the organics in the water are reduced the problem will go away on it's own. The fish's natural immune system will be able to fight it from there without the need for meds. Be sure as well to fortify what you feed the fish with a good liquid marine vitamin containing [acronym="Highly Unsaturated Fatty Acid"]HUFA
If in the main tank, watch for aggression from tankmates.