my luck with powder blues is horrible, I have tried two in my 125, but each time they magically got ick, and i had to pull them out an nurse them back to health in my quarintine tank, and then give em back to the fish store. But hey good luck, they do look beautiful in an established tank, and are very active fish. I would just make sure that the hippo and powdered blue are about the same size, if not the powder vlue being bigger. Hippos, in my experience are kind of shy towards other tangs, but who knows. Good luck!