Well, it seems that some simply won't be convinced. So, with that in mind, and assuming that you'll do the right thing and find larger housing for it when it comes time, then I'm going to recommend a Zebrasoma species, and get it VERY SMALL. I'd also suggest going to a place like fishbase and checking ultimate adult sizes for all tangs, going with that which will top out in the smallest range.
I third the opinions of the 46 & 55 gallon tanks, because it's not just a matter of water quality, but also a matter of letting an animal that has evolved to swim a great deal some room in which to do what comes naturally.