Return Bulkhead so deep?

[Squiddy]

I will have to give that a try. I know that the level in the overflow box and the level in the tank always stay the same. The pump I have should be putting out about 500gph at 5ft which doesn't seem like that much to me. What is the ratio for tank size to amount of water turned over in an hour again? I guess I need to do some more research... just haven't had time.