it is best not to get a tang unless you either have a 6' long tank or want to find it a new home when it gets larger. tangs need a lot of swimming room plus they get fairly large.
also depends on how much rock you have in your tank. i have over 100 lbs in my 55 so there isn't a whole lot of open water for larger fish to swim in so i only keep 4 small fish in it and stock with corals instead.
I know that i'm kinda stealing the thread, but will i be able to keep a white cheek and a orange shoulder tang in a 100 gallon with a 30 gallon sump? Well it's actually 90 gallons cause I'm going to only fill to the brace and including the LR and sand.