If he is in your display showing ich, then guess what, your display has ich. If you remove him to treat him and do not remove all other fish for an extensive period of time then he will just have another outbreak when you reintroduce him. I have great success with two feedings a day, one every other day soaked in garlic to really get him to eat, and a uv sterilizer. Be sure to be feeding veggies along with your regular diet. If the fish is healthy keepin him happy will help him to recover the best way, by his own immune system. You can remove it and medicate, but you'll be risking reinfection upon reintroduction, and even other fish getting infected while the tang is away. This is my opinion from my experience and is not in any way the absolute right way to handle things, just my way.