For as long as I can remember that has been an on going debate on not only on what equipment is the best, but also on the depth of your sand bed.
Some authors, like Robert Metelsky recommend a thickness of about 1-3/4 to 2 inches. This is standard or average depth used in most saltwater aquariums. This is especially popular for giving the aquarium a natural look just like the ocean as well as giving a place for live rock and corals to sit.
Deep sand bed is the invention of Dr. Dean Jaubert. His system includes a Deep Sand Bed (DSB) of about 5â€, a protein skimmer and a plenum. Although most people do not use the plenum in their setup.
The whole principle of a deep sand bed is dependent on â€œsand stirrersâ€ moving through the sand, which keeps pockets of toxic gases which are produced from decaying and dying organic matter from forming. Sea stars, Nassaruius snails mow and plow their way thru the substrate eating uneaten food and detritus along with releasing toxic gases in small quanties.
This is a whole purpose a deep sand bed.
Really then, in my opinon there is no need for a deep sand bed unless you have a plenum. A plenum filter seals the plenum and allows the anaerobic bacteria to convert to nitrates in the tank which in turn converts into a harmless byproduct, Nitrogen.
I suggest anyone wanting a Deep sand bed not to do so unless you will be using a plenum with it.