Actually, I'm not planning to use a pump. Everything will be gravity fed. I am going to put a T in the drain line from the overflow with a ball valve on the side that leads to the fuge. This way I can controll the amount of flow to the fuge, and whatever water that is not handled by the ball valve will back up and go into the sump through the other side of the T. The fuge will sit a higher than the sump so that water will over-flow through a bulkhead down into the sump. Then, the water from the fuge will mix with the water in the sump and be pumped back up to the tank through the return pump.

I think I'll try a small layer of sand with the rubble. This way, I can always add more, or even take the sand out completely if I want to later. Nice to know it doesn't really matter, though. I know that my denitrification is going on in the main tank.