Bare-bottom (BB)? Calcium Carbonate (CC) Gravel? Crushed Shell? Sand? Deep vs. Shallow? With all the choices that are out there, no wonder new hobbyists are often confused about which substrate to use for their marine aquarium. In this thread, I shall attempt to address pros and cons of each of the available substrates.
Please remember however, that the choice of substrate must be balanced with two things; Aesthetic value to the viewer and biological value to the livestock. Some fish and inverts require a certain type or depth of substrate, and proper research should be done to ensure that the hobbyist is correctly matching the substrate with the needs of the aquarium's livestock. Sand-sifting gobies such as the Diamond Watchmen and Yellow Shrimp gobies should never be placed in a BB or CC gravel tank, and many wrasse species require a Deep sand bed (DSB) so that they may bury themselves at night or when they perceive themselves to be in danger.
Bare-Bottom (BB): Bare-bottom refers to an aquarium that has no substrate at all. The bottom remains clear, and allows the hobbyist to easily clean up any left over food or debris which may be floating around, using a net, siphon, or state of the art filtering system. This lack of substrate is preferred by those hobbyists who prefer a clean, sterile look in their aquarium, and is often recommended for a quarantine tank. While maintenance is easy, it must be done regularly as debris blowing around the tank can be very unattractive. The lack of substrate means that the hobbyist is gaining no natural filtering benefit from the substrate and is unable to keep sand-dwelling inverts or sand-sifting fish. Additionally, Extra care must be taken when building rock structures or when working in a BB tank as there is nothing to absorb the impact should a rock wall collapse. And finally, refugium lights on a reverse cycle can shine up through the glass bottom, causing the aquarium to be lit from below; therefore individuals may wish to consider painting their bottom.
A study conducted on the denitrifying effects of sand beds noted what appeared to be a correlation between substrate depth and aquarium deaths; with death rates up to twice as high in shallow substrate tanks, and a word of caution that "it doesn't get much shallower than bare-bottom." While I've attached a link to the article, I urge caution in reading too much into the study, as this was not the focus of the actual research.
Feature Article: An Experimental Comparison of Sand bed and Plenum-Based Systems: Part 2: Live Animal Experiments — Advanced Aquarist's Online Magazine
Calcium Carbonate or Dolomite Gravel (CC): Commonly referred to simply as CC, calcium carbonate gravel used to be the most common substrate in marine aquariums, but has since fallen out of favor among hobbyists, although it is still used by some. Larger particles (usually 5 to 20mm in size) of calcium carbonate or dolomite are placed in a layer from 1/4" to 1 1/2" deep over the bottom of the aquarium. The primary reason for its fall from favor is that many believe that CC acts as a trap for uneaten food and debris and turns the substrate into a nitrate factory. Others simply do not care for the look of CC. If the CC bed is properly maintained through regular siphoning and cleaning, it is no more of a nitrate factor than any other substrate. Calcium carbonate and dolomite are both hard substances that break down slowly over time releasing calcium carbonate into the water, helping to maintain calcium levels and buffer pH in the marine aquarium. Calcium carbonate is the same substance from which many marine inverts, such as snails and many species of coral build their skeleton. Dolomite, although similar in appearance to CC is less soluble and breaks down slower as a result.
(Photo courtesy of Paul Baldassano)
Crushed Shell (CS): Another substrate that has found some small amount of favor among hobbyists is crushed shell. Typically composed of the shells of assorted types of shellfish, CS is generally smaller than the particle size of CC and is generally more aesthetically pleasing to hobbyists, especially those who are familiar with shell-covered beaches. Like CC the CS breaks down, releasing calcium carbonate into the water, however, because it is not as hard as CC or dolomite, it breaks down and dissolves much faster. Additionally, the fine shell pieces in the uppermost layer quickly color up with coralline algae, just as the shells of snails and hermit crabs in your aquarium do. Like the CC if not properly maintained through regular siphoning, the crushed shell can become a nitrate factory as debris and fish waste break down.
Sand: Generally the favored substrate of today's modern reef aquariums, many feel that sand gives the marine aquarium a much more natural look. Additionally, the sand bed provides a place for the growth of micro-fauna 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 is available in a variety of colors (from pink to black), size (very fine "sugar sand" to large grains) and name brands. It is generally recommended to use aragonite sand, so that it, like the CC can bread down slowly over time adding calcium carbonate to the water and helping to buffer pH. Silicate-based sand (usually referred to as play sand) 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 sand bed requires the introduction of bacteria and micro-fauna through the introduction of true Live Sand (not the bagged stuff purchased off the shelves from your LFS which at best contains only bacteria). Some of the disadvantages of a sand bed include sandstorms in the event of too strong of a current within the aquarium and sand being dropped onto LPS polyps. A sand bed 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 sand bed. Some critters, such as Diamond Watchmen gobies and sand-sifting sea stars, can quickly consume all the micro-fauna and bacteria in the sand bed in a smaller tank, thus causing the sand bed to become inert and no longer function for biological filtration. Once they have consumed all the micro-fauna, these animals will often slowly starve.
Shallow vs. Deep Sand beds: Generally speaking, 1/4" to 2" is considered to be a shallow sand bed (SSB) and 3 1/2" to 6" is considered a deep sand bed (DSB). Generally the area between the two 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 the sand bed is disturbed.
(Your photo of a shallow sand bed aquarium here)
It has generally been accepted that SSB, like CC, maintain oxygenated water throughout the sand bed and therefore must be maintained by siphoning, generally on a monthly basis. While DSBs on the other hand have an anaerobic area (typically in the bottom inch or two) very similar to Live Rock, in which anaerobic bacteria are able to establish to help break down nitrates. The DSB requires little to no regular maintenance (some do regularly siphon or stir the top inch or so). Recent research however indicates that both shallow and deep sand beds function in converting organic waste into free nitrogen gas. I've attached a link to the two articles that report these finding below.
Feature Article: An Experimental Comparison of Sand bed and Plenum-Based Systems. Part 1: Controlled lab dosing experiments — Advanced Aquarist's Online Magazine
Feature Article: An Experimental Comparison of Sandbed and Plenum-Based Systems: Part 2: Live Animal Experiments — Advanced Aquarist's Online Magazine
Alternative Materials: A growing number of individuals have also began experimenting with a number of different alternative materials in place of going bare-bottom. Starboard or Corian is one of the more popular alternatives. These materials have similar benefits to BB tank, in that they are easy to siphon off detritus and wastes, and one never has to worry about stirring up sandstorms or clearing sand from LPS polyps. They have the additional advantages of having something in place to absorb a heavy impact in the event of the collapse of rock work, and they lack the sterile look of a BB tank. The lack of substrate however, again means the hobbyist is gaining no natural filtering benefit from the substrate, and is unable to keep any sand-dwelling inverts or sand-sifting fish. Additionally, as with the BB, regular maintenance is required, as accumulated debris blowing around on the bottom can be unattractive, as can the board itself. Further as with the BB, refugium lights on a reverse cycle can shine through from the bottom, so consideration should be given to painting the bottom of the tank.
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