Yes good point Steve, it's the depth that's really the debate. Marine aquarium magazine has a huge article on sand beds and it can be summed up like this.
Sand beds of 1.5-2 inches deep provide as much de nitrification as a sand bed of 4 inches and deeper. The reason if I understood the article was, as long as you use a porous sand IE aragonite or coral (eeek) you get the anoxic regions with in each particle of sand. I'll try to find the article and post some of the highlights.
Doug
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