Perhaps this article will help:
An Introduction to Deep Sand Beds for the Natural Marine Aquarium Part 1 ... only part By Anthony Calfo
http://www.wetwebmedia.com/deepsandbeds.htm
From this article:
"Plenum Or No?
Since the popularization (and misapplication) of the Jaubert-style plenum for deep sand bed methodologies in the early 1990's, much has been written, debated and revealed about the use of this feature. For those of you new to the issue, rest assured that there is little you truly need to know as a casual aquarist. A plenum is a physical water space underneath a deep bed of sand. It is used to create a hidden, dead space of static water to facilitate the diffusion of nutrients and other vital components of biological processes through the substrate. The premise here is that the biological faculties we seek to harness for natural filtration in live sand can be supported and encouraged by this feature. In the bigger picture, this may indeed true. Some of the challenges of employing this technology in the past were understanding and adapting it from the original recommendations that trickled into popular aquarium literature. It seems that at least some of the early systems incorporating this strategy were very large semi-closed or open systems (fresh flowing seawater) with extraordinary depths of sand that cannot easily be incorporated by home aquarists. Without getting too involved in the "how's" and "why's" of the matter, let us summarize that the plenum methodology has not been demonstrated to be exceedingly useful or particularly harmful. Most aquarists find that there is little difference with or without a plenum for a deep static bed of sand in a healthy home aquarium system. This should bear no reflection on the validity of the methodology, but rather illuminates that the adaptation for home aquarists, especially with smaller aquaria, may have little impact. Admittedly, there are no hard and fast rules here. You may have an interest to experiment with the strategy and are encouraged if so. Just know that having a plenum is not critical to success with deep sand bed methodologies.
One last mention of the improper implementation of plenum and deep sand bed strategies collectively. We should like to dispel the most common corruption of the application for those interested to know or try it. Severe criticism of their use has faulted them for becoming "nutrient sinks": trapping and accumulating detritus to levels that cripple water quality and fuel nuisance algae growths. The reality of the matter may likely be that an incorrect application of the technology caused the rift. As aquarists, we too often have inadequate water flow, which prevents detritus and organic particulates from being properly exported by protein skimming and other filtration dynamics. In turn, excess detritus settles in pockets and migrates deep into the substrate. Furthermore, course sand and gravel is still quite popular and allows particulates to settle and accumulate rather easily. The killing blow to a flawed application with course substrates in weakly circulated aquarium is the unfortunately popular employment of intermediate depths of sand at 1"-3" (25-75mm). In this mid range, the sand is often too deep to be wholly aerobic and yet not deep enough for efficient denitrifying faculties. As such, the two dominant (and desired!) biological populations are restricted if not excluded at large and the sand bed may become a dead zone... a nutrient sink. However, intermediate sand depths can be maintained successfully (often, in fact!), but require due diligence with regular sifting naturally or mechanically (by the aquarist or by creatures in the aquarium), strong water flow in the tank, realistic bio-loads, etc."
