<a href=showthread.php?s=&postid=11419836#post11419836 target=_blank>Originally posted</a> by sirreal63
If you want a dsb to process detritus it will never work correctly, the surface area is too small. Rocks cover too much of the sandbed and the fauna does not move into those areas easily. A shallow sand bed works just as well for processing detritus as a deep one...but with much less danger of nutrients being released into the water column. A dsb works for denitrification from the water column where there are anoxic regions with anaerobic bacteria. This is why the remote sump dsb or in a bucket works (in theory, I am not positive it will make a huge difference) as the bacteria function much like a denitrification coil.
In any event I would not have any sand deeper than a few inches where it can be disturbed as this is where the danger is...releasing nutrients into the water column. My first tank crash came from this happening. An established dsb and an engineer goby is not a good idea.
I second what sirreal said,
There is simply no biological way to adequately process detritus in our tanks. In nature, the end result is mineralization... not going to happen in our reefs. The only option is physical removal: skimming, filter socks, siphoning. DSB critters are just a life cycle away from becoming detritus themselves, they don't actually remove anything.
However DSBs do work well for denitrification, whether in the tank or remotely, it does't matter where because NO3 is held in the water column. DSBs are also much easier to setup and maintain when your using them strictly for denitrification. There is no need to worry about critters or varying grain sizes. You just need a fine grain sand (I just use silica because it cheap and won't "cement" up) at a depth adequate enough to produce an anaerobic zone at the bottom.
Eventually, a DSB like this will "fill up" and begin to leach nutrients primarily phosphate back into the system. The secondary function of the DSB is to work as a nutrient sink, as it fills up with detritus and traps organics and other nutrients, however, IMO this is not a helpful tool. In order to prevent a DSB from "filling up" as quickly you need to create a swift unidirectional laminar flow across the surface of the sand to prevent detritus from settling and working its way into the DSB. This is where remote DSBs have their advantage because you can easily control the flow regime. Still, most people setup remote DSBs the same way they set them up in their main tanks so they "fill up" just as fast. In the main tank most of us shoot for broad random flow. This combined with all the LR will always create pockets and traps for detritus to settle.
Currently, I don't have any DSBs in my system. I removed the DSB compartments I had in my sump to make room for a larger in-sump skimmer. My bioload is small and I feel I have enough LR to handle the denitrification, plus by removing organics the upgraded skimmer gives me less nitrate to start with. However, when I piece together my dream tank in a couple years, I will utilize remote DSBs for denitrification. I plan on daisy chaining 3 or 4 together so that I can rotate replacing the sand in one about every few months. The water on the surface of the sand will only be about 1" in depth. I will likely use some sort of spray bar to create a swift uniform unidirectional flow across the entire surface of the sand and spill over full length overflow on the other side. this will basically "sweep" the sand surface and prevent detritus build up.
If your looking for super clean water (keeping solely SPS) I think bare bottom tanks are the way to go. Personally I can't stand the look so I keep about 1/2"
