I can't seem to get my powerheads placed well. I have 4 k4's in my 180, Two are on the back glass, pointed straight forward, and two are on the sides pointed across the tank. i've also had them on the overflows pointed forward as well. the problem is, I can't seem to keep them from blasting the sand out of the corners and front centre of the tank. I want alot of flow, but I'd like my sand to stay put!