Tangs are pretty scrappy fish with others of their species. I would think a power blue should be in a larger aquarium than a 90, but I would think it will be fine. That is pretty close quarters for two tangs especially when one is established in the tank. It really depends on the individuals too. I've had 2 yellows, 2 hippos, 1 Kole, and one PB in my 180 for over 18 months now. They scrap once in a while - just the yellows, but never anything bad. I got the Hippos when they were about 1 inch long and now they both are at least 4 inches in size. I doubt there will be much problem in adding one. Only one way to find out.