This is all just my .02. having a dsb without the proper livestock to maintain it doesnt do you much good. Also having a small enough sand grain to keep food and detrius on the upper layers is essential. If food and detrious can penetrate deep into the sand bed and decays the docs will become locked up in the dsb where a skimmer cant remove them and IMO that will eventually lead to problems like sulfur build up from too many bacteria trying to break down not only the no3 but the docs as well. I have taken down tanks where the dsb wasnt maintained right and it reeks like rotten eggs for hours and sometimes days from all the sulfur being released. If you have small grain sand and a good clean up crew constantly scouring the top layers for food it wont become an issue and the anaerobic environment can function normally by producing a healthy amount of bacteria to reduce NO3. I like conchs for cleaning the sand. Two of them work my six foot tank over daily I have sparkling white sand. I know people that also have good luck using nassarius snails.
Back to the point at hand I run a 4.5" sand bed in a 125g 6ft tank. NO3 is always 0 even with a good sized bioload on the tank. My two conchs, various shrimp, nassarius snials, hermits, serpent star, brittle star, and gobies keep the sand free from detrius and left over food.
These are just my observations and opinions im not saying I know everything there is to know about dsb's just stating what I have observed from experiences.
