you should be acclimating for 2-3 hours...at a min...
I wouldn't do this. 2-3 hours is way too long. There's a sticky in the New to Hobby section about how to properly acclimate a fish so that it isn't shocked by the new water conditions and doesn't die of ammonia poisoning in the bag:
http://reefcentral.com/forums/showthread.php?t=1939508
(note Sk8r's "Be sure and be done within 30 minutes!")
And here is another sticky thread in the New to Hobby section about tangs and tank sizes:
http://reefcentral.com/forums/showthread.php?t=1946007
(note sailfin tangs are recommended in a 240G 8' tank)