Isn't a standard 65 gallon only 36"? If so, you shouldn't have him in this tank, because #1 they get too big for your tank, and #2, they need a temperate water temperature. They don't last long at tropical temps. If the tank is indeed 36" (and im 99.9% sure it is) then your hippo tang doesn't belong there either. With those its not volume, but surface area.