Powder blue tangs are known to be ich magnets, and are not nearly as hardy as some of their cousins! I think the trick to keeping Powder blue tangs is to keep them in a large tank, like starfishjackedme said no less than 6'. I'm not sure but I think they grow to around 9" or 10" in captivity, and the should be well fleshed when purchased not thin in the stomach, and head. a good size to get one would be around 3" to 4". Another thing that I think is important is to offer a varied diet of algaes of marine orgin.
Hope I was of some assistance,
HolaCanthus