The sponge you see drawn on the photo is between the two panels of eggcrate. This is a 9" tubular sponge that was originally intended to slide over rigid plastic tubular pipestand-lattice-work of openings for the passage of water. Used in that fashion caused the occasional filling of the overflows, with the possibility of overflow to the room...not a good scene. The sponge captures bubbles that otherwise make it back into the tank. The sponge does not run all the way from front to back side of sump.
Aquarium tubing runs from the pump to a 3-way splitter. From there, two tubes run to the two corner return holes in the bottom of the tank, which have PVC pipe running up the overflows and over the top into the tank, allowing water to flow in about 1/2" below the water surface. the 3rd tube runs to PVC pipe behind the tank which runs up the middle of the back of the tank, over the top, and down to the top of the sandbed. This PVC is drilled every few inches with holes and capped at the end to allow circulation behind the live rock. The holes at the top are smaller than the holes drilled toward the bottom, and they are largest at the bottom. This was done to maintain equal water pressure exiting the holes throughout the length of the tube. The top hole, 1/2" below the water's surface, prevents the water from siphoning back into the sump when the power is off.
A Rio 800 powerhead, hidden in the corner behind the live rock, pumps water straight down PVC to a PVC bar that runs the length of the back of the tank along the top of the sandbed. Holes are drilled in the horizontal bar on the side of the PVC that faces the front of the tank. Again, larger holes are toward the end of the pipe. The end is capped. This sends water across the sandbed and under the live rock. This was done in December '98 and there is still no detritus back there.