What does a DSB ?
As a mostly anaerobic area with slow water circulation, it allow bacteria to transform nitrates (NO3-) to gazeous nitrogen, which escape from the system. So, it's the main role: denitrification. Note that the same phenomenon also occurs inside live rocks, so sand is optional. If you see my previous post, I can definitely say that it works well!
But the problem is that is also stores organic matter (just see water colour when you siphon it), which slowly transforms-->leaking nutriments, ie phosphorus in organic molecules, which are not detected by phosphate tests... but can be used by algae.
When I read the forums, I have the impression that some people see DSB as a magic system which destroy all pollutants. But we know that is physically impossible, things just transform...nitrogen is really removed from the system because it becomes an inert soluble gas. But not phsophorus which always stays in the system as various chimical compounds.
Eventually, calcium phosphate could be an inert form as it is not soluble, at least if you have a high PH and calcium level, but i don't know if this effect was ever proved in an aquarium. So, I wouldn't count on it.
So, if you put sand, don't put much: less dirt will accumulate and as few as 1 cm (1/2.54 of an inch) is fine for denitrification (I observed that on a micro-reef I had at home a few years ago). Don't put any sand under live rocks: as there is no water circulation, it won't be of any use and will go badly anaerobic, with sulfate reduction to H2S (stinks, toxic to fish, sand/rocks in contact become black). And wash it regularly (you will be surprised in what can accumulate in a few months) and/or use fish to do the job.
There are many posts about DSB in the general discussion forum, far more detailled that what I explain here, so you can read them and make your choice.
