I have a sailfin tang that has a strange looking mark behind his right eye that im assuming is some sort of parasite. Otherwise he s in good condition.
any ideas on what it could be and possible treatment?
That is head and lateral line erosion (HLLE). It is a disfiguring, but not fatal ailment. It is typically caused by exposure to carbon dust at some point. I have an article posted here about it, but I’m on my phone and can’t link it for you.
As far as “cures” you’ll hear a lot of them, but only two seem to work consistently; avoiding carbon and moving the fish to a new tank that has never had carbon on it. It may get worse, but eventually will stabilize.