There are several potential benefits and possible drawbacks to having a DSB. Most often their purpose is for nitrate reduction, so that is this article's focus, but that is not their only purpose, nor is a DSB the only means for achieving that goal. Not everyone uses a DSB and many have tried them with poor results. Some do use a DSB and have reported good results for a decade or more. There is substantial disagreement as to why some succeed and others fail. Over the years, some general rules of thumb have evolved, but they should not be mistaken for definitive science. It is up to the reader to reach their own conclusions.
To better understand the anatomy of a deep sand bed, let us first look at a shallow sand bed, or SSB for short.
- Oxic: oxygenated.
- Aerobic: requires oxygen to function.
- Nitrifying: converts ammonia into nitrate.
In all aquariums, decomposition is largely performed by bacteria, but the process can be facilitated by the presence of a "clean up crew", or CUC for short. Detritus (waste) and other organic matter is first eaten by the CUC of crabs, stars, hermits, and snails. The smaller particulates they produce are then further broken down by copepods, other benthic organisms, and worms. The remaining dissolved organics are then converted by the "nitrifying" bacteria, from ammonia (toxic), to nitrite (less toxic), to nitrate (least toxic). All of this takes place within a layer of sand oxygenated by moving water, termed oxic, and the bacteria there require oxygen to function, termed aerobic. In a shallow sand bed this is where the process ends. The nitrate simply accumulates in the water column to be removed by ritual water change.
In a deep sand bed, there are another type of bacteria, termed anaerobic, that require a depleted oxygen environment to function. Among these are the "denitrifying" bacteria that convert toxic nitrate into nitrogen (which is mostly inert). The primary objective of a DSB is to provide a layer of very low oxygen, termed hypoxic, where bacteria can function anaerobically. The potential harm is in creating a layer completely devoid of oxygen, termed anoxic, where "reducing" bacteria can convert sulfate into hydrogen sulfide (rotten egg smell). This and other toxins can dangerously accumulate in a sand bed that is too deep or not properly maintained.
The prevailing wisdom is that the worms and benthic organisms are vital to maintaining a healthy DSB. In addition to cleaning the sand, it is believed their gentle agitation of the bed helps deliver nutrients to the bacteria while preventing truly anoxic conditions.
- Benthic: surface and near sub-surface sand bed layer.
- Hypoxic: low oxygen.
- Anoxic: no oxygen.
- Anaerobic: requires depleted oxygen to function.
- Denitrifying: converts nitrate into nitrogen.
The full benefits and challenges surrounding deep sand beds are still a matter of some debate, so it is important to point out that the conversation is often confused by competing terminology. Environmental scientists often borrow the term anoxic (labeled in blue) to mean extremely hypoxic, and anaerobic (labeled in blue) to mean truly anoxic.
Here are some general rules of thumb for maintaining a DSB. It should be at least four inches deep but no more than six, consisting mostly of fine grains, sometimes called "oolite" or "sugar-fine". Keep the benthic and worm populations healthy by avoiding Sand-Sifting stars, most crabs, and limiting hermits. Occasionally rejuvenate these populations with fresh liverock or true livesand from a well established aquarium, as this may be key to long term success. The sand bed should only be disrupted very gently over time. Brittle and baby stars, as well as Nassarius and Cerith snails, provide a slow and beneficial agitation of the sand, but vacuuming should be performed with great care, if performed at all. Remember, a deep sand bed is a living thing that must be kept in careful balance.
General Rules of Thumb:
- 4" to 6"; fine-grain; do not disturb or disturb with care.
- Helpful: Brittle & baby stars, Nassarius & Cerith snails.
- Unhelpful: Sand-Sifting stars, most crabs, too many hermits.
- Rejuvenate benthic and worm populations for long term success.
(Jun,2009) Author & Illustrator: Whys. The following credits, listed in alphabetical order, are for collaborative work only and should not be assumed as endorsements of this article. Technical contributions: capn_hylinur, fsn77, jenglish, MattL, tmz, WaterKeeper. Additional peer review: adtravels, Biologist, luther1200, jasonrp104, Nanook, rishma, Sisterlimonpot, thegrun, therealfatman.
I've placed "Nitrifying", "Denitrifying" (or "Facultative"), and "Reducing" in quotes, because I believe these are the best words to use in order to prevent misunderstanding when discussing the DSB.
Hope this helps.
Edited by Whys, 29 February 2012 - 11:33 PM.