In a 150g you could add another tang no problem, one that I would reccommend is the powder blue, they are really pretty and don't get too big, but you can get whatever kind of tang you want to just make sure you have lots of LR for them to swim though because they like caves and stuff like that in the LR. Like the above post mentioned feed the tang lots of greens like lettuce, seaweed, ect. and also to keep it healthy feed him garlic soaked foods or vitamin soaked food to help him to keep his immune sytem good. Anyway I was just asking what size your tank was becuase a lot of people put tangs in too small of aquariums, anyway my tank is a 26 gallon.