I have a cube 36x30x30, the extra six inches are for the overflow that is along the back wall. For the return I use to have a mag12 but was unhappy with the heat production. I have since switched to a tunze 6100 on an osciwave and couldn't be happier. I have seen my tank with the overflow in the center and it has the advantage of being able to view from all sides, whereas mine is only three. On the other hand, my aquascaping is such that I have a school of blue chromis and they never fight. There are also no dead zones in the tank and the lighting is made easier.