Tangs are more trouble than they are worth in my experience anyway. I recently took a hippo tang I had for over five years into the fish store, he was eating too many corals and causing too much trouble for the pretty flash of blue he added. Once they get large you have to feed them so much food, it was really unbelievable how much food I was giving this hippo, and still he seemed ravenous and would clear out a patch of zoanthids in a day. He had this neurotic personality too, and would sometimes hide when I fed the tank, meaning I had to feed even more to keep him in good condition.
Now in the 120 we just have the sailfin tang, and even though he seems a little more well adjusted than the crazy hippo, I still feel a lot of guilt about how little swimming room he has. He is about 8 years old and a very big fish. Someone who got him as a baby gave him to me when he outgrew what they had. So I feel a little better that he is not cramped in a smaller tank, but if I ever find anyone with a 400g or something that wants him, I would give him away. He just has no room, sometimes when I watch him he reminds me of a tiger in a cage, pacing. Not exactly the relaxing scene you want when you sit back to watch your tank.