If you are even remotely involved in salt water aquarium keeping, I am sure you've heard the word "plenum". A plenum, also known as the Natural Nitrate Reduction (NNR) or Jaubert system, is basically a natural approach to filtration. The emphasis is on natural, biological filtration instead of the traditional "gadgetized" methods of filtration.  

A "plenum" is basically a space or void. In aquarium lingo, the plenum is a space under the substrate, which separates a thick sand bed from the bottom of the tank. This space becomes filled with water. Organic waste in the water gets transported into the plenum by way of diffusion. As the water diffuses through the sand bed, nitrifying bacteria in the upper layers of sand, utilize the oxygen in the water and break down the organic waste from Ammonia, to Nitrite, and then into Nitrate. The Nitrate is then transported into the plenum via the oxygen reduced water. The Nitrate concentrates in the plenum and slowly gets broken down into harmless chemicals by denitrifying bacteria that live in the oxygen poor environment of the lower levels of the sand bed. 

Conventional methods of filtration fall short at the Nitrifying stage of the process. This causes Nitrates to build up in the aquarium water. Although Nitrate is less harmful to fish and invertebrates than Ammonia and Nitrite, Nitrate can rapidly accumulate to harmful levels. The aquarist is then forced to do frequent water changes in hope to reduce the concentration of Nitrate. The plenum on the other hand, breaks down the Nitrate, maintaining better water quality without the need of excessive water changes. In most aquariums with a plenum system, as in my own, Nitrates can be maintained at undetectable levels which more closely resembles the water quality in nature.

The thick sand bed above the plenum not only helps maintain excellent water quality, but it also adds biodiversity to the reef. The sand bed harbors various species of tiny crustaceans, worms, snails, and starfish that will help to break down uneaten food that would otherwise rot. In addition, these critters help recycle fish and invertebrate waste. These tiny creatures will also serve as a natural food source for the main aquarium inhabitants. 

More recently, many aquarists feel that the plenum is not necessary, and a thick sand bed alone will yield the same results. Since I have not tried such a system, I cannot comment. I can say though that I have corresponded over the net with many reefers that have successful system employing a thick sand bed without the plenum. I may try eliminating the plenum in my next reef tank, but first I must do more research. I would suggest the same for you, or any other aquarist that is planning a system. Whether or not you decide on installing a plenum in your reef, I urge you to do as much research as possible before getting your feet, or hands wet.