Deep Sand Bed

Worm within it's burrow in a sand bed.

Introduction

A deep sand bed (DSB) is a sandbed of 10cm or more in the bottom of the tank. The optimal average grain size for biodiversity is around 0.125mm, however grain size (within reason) has not been shown to affect the other features of a DSB.

A deep sandbed is often considered by aquarists once they understand nitrate and are looking to reduce nitrate. Nitrate can become problematic for species within our aquaria and can build up in a tank unless the aquarist provides some method of nitrate consumption/removal. A DSB has areas of low to no oxygen in its lower layers and this provides an environment for anaerobic bacteria. As there is little or no oxygen, these bacteria use nitrate as a substitute for oxygen and the product formed after consumption is nitrogen gas (harmless). It also has the benefit of the nitrifying (aerobic bacteria) converting the ammonia to nitrate nearby in the upper layers, and is thought to increase efficiency.

Suitable Substrates

Sand beds can use any sand type of substrate. What is important for the substrate used is that it is stable/inert (does not dissolve and release unwanted material into the water) and of the appropriate particle size (<1-2mm). Suitable materials include:

• coral sand
• crushed marble
• calgrit (crushed limestone)
• silica sand, contrary to popular belief silica sands do not increase diatom blooms, see Silica Sand by Rob Toonen.

Using a calcium carbonate substrate (i.e. coral sand or crushed marble) can also have the added benefit of buffering alkalinity and adding calcium. However, due to the small amount that will actually dissolve, volume of the sand bed etc, this will have negligible effects on the calcium/alkalinity concentration within the water. It is also not as effective as other methods of calcium/carbonate buffering, often taking longer times to contribute significantly to buffering capabilities. Therefore, another method of calcium addition is required.

Gallery

• 

  Worm within it's burrow in a sand bed

• 

  View of deep sand bed, showing worm burrows and algae / cyanobacteria growth

• 

  Tank with adjoining refugium containing deep sand bed and mangroves.

• 

  Coating of diatoms

• 

  Coating of cyanobacteria

Benefits

The deep sandbed however, has many more benefits to an aquarium than just de-nitrifying potential. It can provide:

• extra surface area for nitrifying (aerobic) bacteria,
• habitat for fish that enjoy sandbeds, like wrasses or sand sifting gobies,
• habitat for detrivores - [invertebrates]] that feed off waste such as worms and brittle stars, and
• indirect a food source for corals and fish as the detrivores reproduce.

Care and Maintenance

A DSB must be kept live. Live sand refers to a sand with lots of critter in it, detrivores and 'pods that constantly turn over the sand. This requires yearly "reseeding" with seeds from fellow aquarists, local bay, or even aquarium store. Fresh live rock often comes with many critters that help a DSB.

Critically, one should never siphon a DSB.

Possible Issues

A common accusation leveled at DSBs is the buildup of hydrogen sulphide pockets, which can occur when areas of the sand bed go anaerobic. This can lead to deaths in the tank when the sand is disturbed and the hydrogen sulphide is released into the water column. This has been proven to be non existent in live sand beds that are properly maintained. And even if it is present in the sand bed, this is not an issue in itself. A sandbed can have regions of hydrogen sulphide production, which appear as black / grey areas, without any problems. They typically appear when the sandbed is initially established, moved or more added. Within a couple of weeks the region will start to reduce in size and eventually disappear.

The other potential issue is the existence of 'old tank syndrome' or the fact that DSB's might go 'stale' after 5 years or so. There is far from any conclusive evidence of this, and any attempt at explaining what might actually be happening is not creditable. Many believe that these rumours are the result of lazy aquarists looking for an excuse - there are many examples of DSB's running for much longer than 5 years.

One option some people take is to go a remote DSB, that is one in the sump or the refugium that can be removed if necessary with ease. This has been shown to work ok (although its only a recent trend).

Resources

• Dearest Mudder.... The Importance of Deep Sand by Ronald Shimek - RonShimek.com
• An Introduction to Deep Sand Beds for the Natural Marine Aquarium Part 1 by Anthony Calfo - WetWebMedia
• An Experimental Comparison of Sandbed and Plenum-Based Systems. Part 1: Controlled lab dosing experiments by Rob Toonen - Advanced Aquarist's Online Magazine
• An Experimental Comparison of Sandbed and Plenum-Based Systems: Part 2: Live Animal Experiments by Rob Toonen - Advanced Aquarist's Online Magazine
• How Sandbeds REALLY Work by Ronald Shimek - Reefkeeping Magazine
• The Old Becomes New, Yet Again: Sandbeds and Vodka by Eric Borneman - Reefkeeping Magazine
• What lives in live sand? (Do you really want to put your hands in this stuff?) by Ronald Shimek - Aquarium Frontiers Magazine
• Sand clumping- the How`s and Why`s by Mike Kirda and Shane Graber - Reefs.org
• Reef Aquaria as Ecosystems by Ronald Shimek - Reefkeeping Magazine
• Deep Sand Beds by Simon Garrett - Reef Eden
• Bristleworm Spawning and Sandbed Questions by Ronald Shimek - Reefs.org
• SandCalc, a Windows program for calculating sand bed depths - Reefs.org