sailfin tangs need very large tanks...they get over a foot long. i personally do have one in my 90. i actually didn't want it at first. but it is still pretty small, but i have a lot of swimming room and all the other fish in the tank are small (chromis, firefish, gobies, ocellaris clown, etc) and i have a huge tank upgrade in the near future. i graduate in may, buy a house by december '08 or january '09, order my 375 gallon tank (i might go smaller though), and i should have it up and running by the end of '09. but even if that falls through for some unplanned reason, i have plenty of places to put this guy.
but if you already have a yellow tang, then don't do it. that's too much for a 75.