I personally wouldn't add any tang to a tank that was less than 6 ft. long and at least 125g, with certain species requiring even more space. Some say that kole and yellow tangs do OK in a 4 ft. tank. There are many differing opinions on this issue. In my experience, it is only a matter of time before a tang in a small tank starts to suffer from some sort of confinement stress (namely poor health or aggression).