Sand: Generally the favored substrate of todays modern reef aquairums, many feel that sand gives the marine aquarium a much more natural look. Additionally, the sandbed provides a place for the accumulation of microfauna and bacteria which aid in the biological filtering of the marine aquarium. Sand is used to cover the bottom of the aquarium to various depths, and now comes in a variety of colors (from pink to black), size (very fine "sugar sand" to large grains) and shape. It is generally recommended to use aragonite sand, so that it, like the CC can break down slowly over time adding calcium carbonate to the water and helping to buffer pH. Silicate based sand (usually used for sandboxes and playgrounds) should be avoided; it compacts readily, creating a concrete-like substrate, and releases silicates into the water which can cause diatom blooms. Properly setting up any sandbed should include the introduction of bacteria and microfauna through the introduction of true Live Sand, not the stuff that is purchased in bags at your LFS (which contains only at best bacteria). A sandbed is required to properly maintain certain species of fish and invertebrates, however care should be taken in the selection of livestock for the marine aquarium with a sandbed. Some critters, such as diamond watchmen gobies and sand-sifting seastars, can quickly consume all the microfauna and bacteria in the sandbed in a smaller tank, thus causing the sandbed to become inert and no longer function for biological filtration. Additionally once they have consumed all the microfauna, these animals will often starve.
Shallow vs. Deep Sandbeds: Generally speaking 1/4" to 2" is considered a shallow sandbed and 3 1/2" to 6" is considered a deep sandbed. Generally the area between is considered to be an unsafe zone, not quite deep enough for anaerobic bacteria to establish and carry out denitrification, but too deep for aerobic bacteria to be present, thus allowing the build-up of toxic wastes in the form of hydrogen sulfide gas which can be released if/when the sandbed is disturbed.
It has generally been accepted that shallow sandbeds, like CC, maintain oxygenated water throughout the sandbed, and therefore must be maintained by syphoning, generally on a monthly basis. While deep sandbeds (DSB) on the other hand have an anaerobic area (typically in the bottom 1") very similar to Live Rock in which anaerobic bacteria are able to establish to help break down nitrates, and requires no regular maintanence. However, research indicates that both shallow and deep sandbeds function in converting organic waste into free nitrogen gas. I've attached a copy of the two articles that report these findings below.
http://www.advancedaquarist.com/2005/6/aafeature#h5
http://www.advancedaquarist.com/2005...?searchterm=an experimental comparison of sandbed and plenum-based systems. Part 2
