Your best option is water changes. Even if your parameters are "all fine", opportunistic bacteria is fed from DOC
which is at the beginning of the nitrogen cycle. Reduce the DOC
and the fish's own immune system should take care of the rest as long as the foods fed are properly nutritious. Wouldn't hurt to soak the foods in a good liq marine vitamin beforehand.
A few large water changes with well aged/aerated SW
every second day for 7-10 days should see some improvement. Meds are rarely if ever needed for something of this nature unless advanced or parasitic which this does not sound like so far.