Garlic and ich is another myth I think. Although it does help to get the fish eating, which is a good thing when it's stressed, but there's no evidence it helps to cure or prevent ich.
I've recently had two fish get tiny white spots. An anthias, which had them all over, then died the next day, and my Royal Gramma which just got a few on it's face, then they disappeared and never came back. The clowns never had a single spot.
There are thousands of possible marine parasites, that can look like ich, but aren't. I suspect mine was one of those.
In summary, I'd wait and see if it spreads to other fish before doing anything drastic. The only real proven cure for ich is to take all the fish out and use copper, and leave the DT fallow for 3/4 months.
You could take the tang out and treat in a HT if you can get one set up.
I'd suggest the first, simplest thing would be to give it a fresh water dip. If it's external parasites, they should start to fall off. If that happens, you'll probably need a few dips over a week to get rid of them all. Putting it in a HT will help isolate it, and keep the numbers down in the DT.