A few places. SOme put it just below the top intake slightly angled to the right (as you are looking at the tank from the back). Another spot is just under the other return. Point one return towards the surface and the other to the left (as in towards the main display). I actually have mine on the left side of th tank (as you are looking at it from the front) angled towards the front glass to create a circular motion to the intake grates.
It can go just about anywhere..all you have to remember is that you want flow to all areas of the tank.
I have mine on the left side of the tank next to the back wall just under the return. Its angled to about middle way of the tank. I can't turn it anymore to the front (right). I believe the circulation pump i have is a 240.