First of all, I have to agree with fishymissy. One thing that I feel is important to keep a tang in a 55 gallon is to not have that "wall of rock". Sure tangs like to graze on algae, my tank makes planty. But my regal will eat mysis and seaweed also, so feeding is not a problem. With not as much rock, I have about 55 lbs in two stacks with a valley in the middle, the tang seems to like having a little extra room to move. She is a vibrant blue, eats well, and loves to show out for me.
One other thing I will make a suggestion for tangs. I bought my regal at only about 2 inches long, if that. As a baby, it is easier for her to adjust as well as she will be comfortable in ym 55 gallon longer. Sure, ina year or two I may be ready to upgrade the tank larger anyway, and she will be happy. But as a baby with planty of open water, I think a tang in a 55 is just fine.