I would suggest maybe a purple or yellow. I had a NASO,Powder Blue, and a Kole (not all at once) in my previous tank which was the same dimensions as yours and they all died because the tank was not wide enough,48". The purple and yellow survived. They died from stress according to others in RC. I now have a 150 60" wide and only added my Atlantic Blue, but not any other types of tangs. I learned my lesson the hard way. just my personal opinion.
Prime