giant alien come to earth, scope you up and bring you back to their planet, put you in a glass box (2000 sq ft living space) filled with oxygen, cuz their planet's air is not suitable for you, feed you daily, (sometimes twice a day if they are in a good mood), change out your waste once a week, etc...
Now if you are telling me that is a better life than living on earth where you have to work to stay alive, might get robbed and stabbed, shot, hit by a car any day. Then you can tell me the fish is happier in our tanks.
All jokes aside though, to the op, I think if you start them small in a 120, you can do it, fish probably won't grow to it's full potential size though. I have seen many 120 tanks with large purple, yellow and sailfin tangs in them. As long as you can be creative with the rock work to reduce potential territory aggression.