We've had our Yellow Tang since sometime in 1999, so going on 5 years. It's body was the size of a half-dollar when we got it -- he's probably about 5" (tail to tip of nose) now.
I truly believe that one of the keys of successfully keeping Tangs (we also have a Purple Tang that is a few months older than the Yellow, and a Regal which we've had about 7 mos) is a varied diet, based heavily in greens/seaweeds.
We feed our Tangs our homemade tank food
They also get (4x / week) - Nori, Dulse, organic romaine & baby spinach, all "marinated" in Zoe, on a clip.
Every once in a while (1-2x/week), we'll offer Spirulina flake, or OSI spirulina nibblers.