surely even 75us gal is too small for a tang, the sites i have been reading recomend 100uk gals for any kind of tang. it seems silly to me to to buy a fish that you will either have to upgrade your tank or re-home it when it outgrows your tank.
even the most basic of marine books tell you to reserch the adult size of the fish you think you wan't to keep. also it should be considered that, not long before you buy the fish it had the whole ocean to swim in. i know there is no tank that can match the size of the ocean but how would you like to live in a 12ft cube, when yesterday you had the world to explore?