Good luck getting this piece of junk to work even as a prefilter box. I threw mine away. First off, it is very difficult to fit it over the pump. (BTW, there will be no room for anything else in there. Oh, and reef rubble will clog your pump intake.) Secondly, the slots are inadequate. If you are using the best recommended pump, you can count on it running dry. The slightest drop in your tank's water level will result in inadequate inflow and the box will empty, leaving your pump to run dry until either you notice and get the water flowing again or until it burns out.
If you haven't opened it yet, I'd suggest you return it for a refund.