What kind of fish is it and how long have you had it? What are the tank specs?
If an established fish, the main causes will be high DOC
, poor water quality, improper diet and/or aggressiveness in the tank.
Typically increased (larger) water changes with well ages SW
and improved feedings with vitamin fortified foods will help with clearing the problem up on it's own. If that is not effective, then treating in a seperate QT
with an antibiotic would be best. There is nothing that can be safely used in the main display to properly treat these types of problems chemically.