We've had a Powder Blue for over 3 years--first in a 120G tank, since July in a 300G. We feed it spirulina and vibagro mostly--no sea veggies, nori, etc. It eats like a pig, is very fat, has great color, and has grown considerably since we got him. Every few months it starts to rub on the sand for a day or two, then stops; I suspect this is the result of a minor ick outbreak which the fish seems to be able to handle on its own quite easily. It follows me whenever I come near the tank, chases the magnet when I'm cleaning, and is in general a very gregarious fish.
It was a major bully in the 120, but not so in the 300. (We have 2 other tangs, a copperband butterfly, a coral beauty, a flame hawk, bi-color blenny, two skunk clowns, and a bunch of "convict" damsels in the tank with it.)
George