If you're talking about the regal tang or hippo tang, (probably) the really little ones are very hard to keep alive IME. I think they're much less stessed in a group, I've almost never seen one by itself less than a couple inches long that was healthy. I saw a couple the other day at a store and was tempted, as these were the smallest I've ever seen and I'm looking for only tiny fish for my tank. Sure enough, they were both scratching away and the colors looked kind of dull, both bad signs.

Keeping the fish stress free as much as possible, alng with all the good fedding advice you've gotten, is the only thing I could add.