I had a blue hippo in my 46 bow front. The hippo was small, but you can clearly tell that they need more room to swim around. Plus they also like a lot of rock to hide when they get scared and also they like to squeeze in between the rock. So if you add more rock, you're going to have less water in your tank, and to tell you the truth, it's not worth it. If you decide to keep one, once you want to get him out of the tank, you'll basically have to remove all the rock, since they are quick and also get scared easily. In addition, tangs in small tanks seem to be more prone to ich.
When i was able to take him out of the tank, i gave it to my friend who's got a 175 bow front, and the fish looked completely different. Acted a lot differently, seemed happier, i really wouldn't do it until you get a bigger tank!