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Volume 17, Winter 2000
Live Sand Beds, Some Whys and Hows
by Ronald L ShimeK Ph.D.
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Live sand beds have become a popular way to simultaneously provide a biological filter, a way to remove and utilize excess food, a food source for suspension feeding animals, and new habitat for reef aquaria. These sand beds have developed from the thin sand layers found in the "Berliner" type of aquarium, but they have their theoretical basis in marine soft sediment ecology. They are an attempt to set up a "slice of life" that functions as a normal sand ecosystem. In our aquaria, the sand bed organism community is probably the closest match to any natural reef assemblage. Fortunately, the construction and maintenance of such a bed is easy and straightforward.
Most reef aquarists make their sand beds out of aragonite sand, but there is no compelling reason to do so. Siliceous sands, such as mortar sand, are generally avoided as these tend to increase diatom growth. Other sands, such as black lava, will work as well, and may make an exceptionally good looking substrate. Aragonite sand beds may be a potential source of calcium and buffer for the aquarium, but if significant bed dissolution is occurring, the system's water chemistry is badly askew and this dissolution won't help much. In general, aquarium organisms don't care about the composition of the sand bed, they just need a specific particle size.
The mixture of particle sizes is very important as it facilitates animai iife in the bed. Optimal performance seems to come from a bed composed of particle sizes with a ratio of 5:4:3. The first group is very fine particles sized between 0.063 and 0.125 millimeters (mm). The second group is between 0.125 and 0.25 mm, and the third 0.25 up to 2 mm. Particles larger than 2 mm should be avoided except as a thin surface layer, no more than 6 mm thick, in areas of high current.
The correct mixture of particle sizes ensures that water percolates through the bed but not too quickly. This results in layers of different dissolved oxygen concentrations in the bed. The uppermost layer, two to three centimeters (cm) thick, has a high level of dissolved oxygen, and contains much of the bed's animal life. The next layer has a much lower oxygen concentration and is where much of the bacterial utilization of nitrogen compounds occurs. The lowest layer, seldom found in beds less than 15 cm deep, lacks dissolved oxygen. This is where sulfate reduction occurs that results in the production of hydrogen sulfide. In a normal sand bed the amounts of hydrogen sulfide produced are minute, but even the slightest bit is evident by its pungent odor. Contrary to popular belief the presence of hydrogen sulfide presents no real danger in an aquarium. Even if the bed is severely disturbed and the odor of the gas is quite strong, it will be harmless, as the amount necessary to cause real harm is fairly large. The gas simply smells bad. Normally it slowly passes up through the bed and is oxidized to sulfate when it encounters oxygen.
(Photo: A mature sand bed about 10 cm deep)
Bed construction is easy and may be done when a tank is set up or after it has been in operation for a while. Except for larger armoring particles that are added last, the particles should be mixed before being added to the aquarium. Don't bother to rinse the mixture because the very fine silt particles are good bacterial substrate and will settle out in a day or two. If they don't, a few hours of mechanical filtration will remove them from the water. Also, don't worry about bed particles fouling the organisms. Sand bed construction mimics a moderate storm in an atoll that would stir up animals in the same way. Animals have had to tolerate this condition for millions of years and have adapted to the process. Rocks can be placed on the sand or partially buried in it for stability. If sand is added to an established tank, all other gravel should be removed first.
Once the sand is in the aquarium bacterial colonization will follow. Introduction of some animals should be done after the bed has settled. There are many ways to obtain a diverse assemblage of animals, and the more diverse the population, the better. Various starter kits containing worms, bugs, brittle stars, and other animals are commercially available. Hobbyists can also trade animals and sand. It is important to remember that once the animals are introduced, it will take months for them to fully populate the sand bed. The tank won't be ready for a full organism load for at least six months.
If bacteria do all the work of changing nitrogen compounds into nitrogen gas, why do we need to worry about the worms and bugs? They are important because they keep the whole system functioning through their feeding and other activities. The adequate functioning of the bacteria is dependent on the continual growth of the population. If the population ceases to grow its metabolism of nitrogen compounds will be severely curtailed and nutrient levels will rise. The worms and bugs continually eat bacteria, thus opening up habitat space for new bacteria. Also, as a result of their movement, water can circulate through the bed, bringing dissolved food to the bacteria.
It may seem trivial that space for bacterial growth is necessary, but, as with all other organisms, the population will continue to expand only as long as conditions are good. As available space decreases, the bacteria quit growing rapidly and begin to secrete a hard material around themselves. This material acts like a glue, cementing filter bed grains together into clumps. This occurs rapidly when the bacterial population is large and the animal population is low. Clumping cuts off large areas of the filter bed from water circulation. This kills the biological filtration in the region and the sand bed ceases to function. The action of small worms, bugs, and other animals prevents an over-abundance of bacteria and keeps the system going.
It may seem odd that the movements of such small animals could be significant to water circulation, but the cumulative action is considerable. Each animal causes the disturbance of about 100 cubic millimeters of filter bed per day. In a well populated sand bed the number of animals is enormous. I have calculated that there are about 100,000 animals in the filter bed of my 45 gallon lagoon reef. A population this large will move about 0.01 cubic meter, or 2.8 gallons, of filter bed per day. In my tank this is enough to turn over the entire bed every week or so.
The water circulation resulting from this filter bed movement permits denitrification. The breakdown of other nutrients also occurs. If there is some way to trap these nutrients, in algae, for example, and then remove them from the aquarium, the dissolved nutrient level in the system can be maintained at a very low level. The best part of this process is that the animals do all the work, except for the occasional removal of a handful of algae from the sump.
There is another benefit to this system. As these animals feed and grow they reproduce and flood the tank with microscopic eggs, sperm, and larvae. I have found that many larval forms are continually in the water of my aquarium, even though I run a protein skimmer. These larvae are coral food. In nature small planktonic animals such as these are the major source of nutrients for small polyp stony corals, many soft corals, and other animals that feed on suspended matter. So, with the help of your friendly worms and bugs, once you feed the fish in your tank you also feed many of the invertebrates, and in an absolutely natural manner.
(Photo: Some animals found in or produced by sand beds)
It is no coincidence that successful maintenance of many corals began with the advent of the "Berliner" system with its rudimentary sand bed. The sand bed produces food for the invertebrates and acts as a biological filter. In a well developed bed the feeding and filtering potential is maximized for the benefit of our captive reefs.
References
Shimek, Ron L. 1999. The coral reef aquarium: an owner's guide to a happy, healthy fish. Howell Book House, New York.
Aquarium Frontiers Online, October 1997. What lives in the sand? (Do you really want to put your hands in this stuff?). http://www.animal network.commsh2/aqfm/1997/oct/wb
Aquarium.Net, 1996. Captive ecosystem energy dynamics and the necessity of scavengers in a marine reef tank.
http://www.aquarium.net/1196/11964.shtml
Shimek, Ron L. September 6, 1998. The why's and how's of sand beds. The role of the benthos in the reef aquarium ecosystem.