FWIW, the other Acanthurus species that I would call my favourite is Acanthurus nigrofuscus, the lavender tang. Everyone and everything I read calls this a plain fish (even wetwebmedia.com) but I saw them in Hawaii and I thought they were exquisite the way they glow purple and pink. Maybe in tanks they are not like this?
At any rate, they are claimed to max out at 8", also known as a peaceable fish.
Anyhow thought I'd share the suggestion if nothing else.
http://www.hawaiisfishes.com/fish_of.../fom_05_03.htm
Although they've mislabeled it as a Ctenochaetus, marinecenter.com shows a picture of what I saw them to look like when I was snorkeling:
http://www.themarinecenter.com/fish/tangs/lavendertang/