your best bet is to remove the fish and treat in a qt .
either with meds or hyposalinity)
have you read much /studied ich?
treatment is really only effective when the ich is in the freeswimming stage.
there are other methods ..using mutiple tanks and moving the fish to a new tank , clean the previous tank and prepare it for the next move.
the idea is to reduce the numbers and exposure to the ich and removing as much of the parasites(cast off)by swapping tanks..I suspect this method is very effective and iif done propery very safe for the fish.
outside of that there isn't much you can do..you can try UV(most of your beneficial bacteria reside on substreat, LR etc.
you can try reef safe meds..the other meds may wind up killling everything in tank short of the fish(and even some of them)
I am treating with UV on my 110 and will try adding some (not so effective) reef safe meds in conjuction with the UV. freaking tang is freakin fast and I don't want to dismantel the reef as the corals are doing great.