It's funny, I've been using an under-gravel PVC grid to return a portion of the return water from my sump in my las two tanks. In my 100g I think there is about 35' of 1/2" PVC with small 1/16" holes facing diagonally down from the pipe. I run mine with a bit more flow, it has a 250 gph pump hooked up to it, where it climbs 5' over the rim of the tank before plunging down below the gravel. The rest of the return goes to the surface via a 800 gph pump to disturb the surface, though a small portion of that runs to my up-flow algae scrubber in a closed loop returning to the sump.