If you're talking a blue tang like in "Finding Nemo", they need a bare minimum of a 6 foot long tank, preferably an 8 foot tank. Bristletooth tangs (and arguably yellow tangs, although opinions differ on them) are the only ones that can be comfortably kept in a 75. The bristletooth family isn't as showy as the Blue Tang, but they have their own understated sort of beauty. Again, research, research, research. You want to make sure that you get fish that are compatible with each other, compatible with whatever inverts you want to keep, and will be able to comfortably live in the tank you have, even as adults.
A common mistake people make is getting a fish that requires a huge tank when it's small, with the idea that they will get a larger tank as the fish grows. Often this doesn't happen, and the fish is just confined to a tank that's too small for it. This leads to aggression, stress, illness, and eventual death. Or else they're forced to rip apart their reef to get the fish out, and that stresses everything else in there to the point of possible illness and death, and can also cause a nitrate spike as detritus is stirred up. Bad idea all the way around. And super easy for you to avoid, since you don't already have the fish in your tank!