First of all, I would like to make sure that the term I have been given is the same meaning to me as it is with everyone else. My reef store owner highly promotes the use of a "plenum". A system in which you use eggcrate sitting on top of 1/2 inch pvc pipe laying on its side with mesh over the top to keep sand from getting down through it and then the sand bed over the top of this. What is the benefits of having this dead water space beneath the sand bed? My buddy and I came up with a solution to having things going down and out the overflows. We did the whole "plenum thing and then we have a drain going out the bottom of the tank. We put a 3/4 inch riser pipe coming up from the drain to the desired water heighth in the tank. Then we took a 2" piece of pvc and cut slits in it at the bottom all around the base about 2 inches long. Then we slid this pipe over the 3/4 inch pipe and cut the mesh around the pipe. So the water goes into the tank from the supply at the top, goes down through the sand, fills the 2" pipe and then the water goes out the riser and down the drain in the bottom of the tank. (note: must keep the top of the 2" open or if you cap it make sure you make an air hole or it will create a syphon.) Are we breaking some cardinal rule by drawing water down through the sandbed?? If you want I can submit some pictures of how this is all going together or you can check out the thread in the breeding forum.. I will be adding pictures to the breeding project post almost every day now. Thank you and any info in this area will be greatly appreciated.