Dearest Mudder.... The Importance of Deep Sand.
By Ronald L. Shimek, Ph. D.
Mud! Why should you put Mud, of all things, in a coral reef aquarium? Well, the simple answer is that that mud will help create an environment that will almost force your corals and other decorative animals to thrive. With some 30 years of experience as a marine ecologist behind me, I can say that THE most important component of a coral reef aquarium is a deep sand bed, comprised of very fine sandy sediments that we can, without any hesitation, call MUD. In this article, I will discuss three things, first, the benefits of a sand bed, then how to set a bed up and, finally, some of the possible problems that you might encounter.
Benefits:
Hobbyists might think that sand beds have no place in a coral reef aquarium, particularly if they are trying to establish something resembling a natural coral reef. However, with some thought I am sure they would realize that most coral reefs are surrounded by sand area, and by constructing a sand bed in our aquariums we merely emulate nature. These beds provide three things. First, they provide a place for processing and exporting some dissolved nutrients. Second, they provide a place to recycle detritus, excess foods, animal feces and other particulate material into useable forms. Finally, they provide a food source for many reef animals. Let's look at each of these functions.
As they do in nature, the sand grain surfaces of sand beds in our systems provide the major substrate for nutrient processing bacteria. The bacterial population is determined by three factors: the total sand surface area; the amount of nutrient available; and the number and effects of bacterial predators. All of these play a role in the development of the sand bed biological filter.
In a given volume of sand, the usable bacterial surface area rises rapidly as the average particle size decreases. For example, a cubical particle 1 mm on a side has 6 square mm of surface area, while the surface area on a particle that is one eighth(or 0.125) mm on a side is a total of 0.09375 square mm. However, in the volume of 1 cubic mm, there would be 512 of the smaller particles, for a total area of 48 square mm, eight times what is found on the larger cube.
The total sediment surface area in even a small tank is impressive, indeed. In my 45 gallon reef tank, the sand bed averages about 4 inches deep, by 12 inches wide, by 36 inches long, for a total of one cubic ft of sediment. I won't bore you with the calculations, but if the average particle size is one eighth mm, and that is a good average size to have, the total sand surface area is about 14,828 square feet or just slightly over 1/3 of an acre. A LOT of bacteria can live with that amount of space!
Although we seldom consider bacteria when we set up our systems, they are exceptionally important to the survival of every decorative fish or coral we add to the tank. Those bacteria are the biological filter of your reef tank, and by their simple existence and growth they detoxify and remove many of the excess nutrients from the system.
One organism's poison is another's nutrient. Fish and invertebrate urine, largely ammonium hydroxide, or ammonia gas dissolved in water, is the primary byproduct of necessary protein metabolism. Ammonia gas, even very small amounts dissolved in water, is highly toxic to animals. Likewise, phosphates are also byproducts of animal metabolism, and although not toxic to most animals, high phosphate concentrations reduce or stop coral growth. The removal of both nitrogenous wastes, such as ammonia, and phosphates is accomplished by bacteria and microalgae which absorb these toxic animal byproducts and use them in their growth as necessary, required, and vital nutrients.
The surface area for bacteria and microalgae in live rock or on other surfaces is insignificant compared to the area in a sand bed four or more inches in depth. The cardinal rule of animal husbandry is that you have to feed animals, and many reef animals need to eat a lot. My article in the February 2001 Aquarium Fish Magazine about the composition of many foods and additives can be used to calculate just how much of the various nutrients you add to your system. As an average the dried foods that I tested had about one half of their weight as protein, which in turn means they have a very large amount of phosphate in them. And, if that was not enough, once the food has been eaten and processed by the animals, they urinate out protein byproducts as ammonia. Simply feeding your fish or corals the necessary food they need to live may boost ammonia and phosphate concentrations several hundred to several thousand times what is normally found in reef water. But, if you have a deep sand bed, a process that is nothing short of miraculous occurs. The bacteria and algae living in the sediments take up the nutrients so fast and so thoroughly, that hobbyist test kits typically may not measure any of the nutrients at all even immediately after feeding.
These nutrients act as food for the bacteria. In a very real sense, the biological filter depends upon bacterial growth. The breakdown of nitrogen compounds to nitrogen gas is done by bacteria growing in the areas of lowered oxygen concentration in the deeper parts of the sediments. At normal reef temperatures, around 82 deg F, some bacterial species will double their population in less than a half hour if they have the appropriate nutrients. This rapid bacterial growth rate causes the release of nitrogen gas which becomes visible as bubbles in the sediments.
Left. Sediments in my 45 gallon lagoonal reef tank showing gas (Nitrogen) bubbles in the sediments; several sand layers determined by oxygen concentration are evident. The bed is about four inches deep. Center. The same tank, several months earlier. Note the coarse material, the GARF grundge, to the right on the sediment surface. The large particles in this acted to reduce the worm access to surface and eventually caused sediment clumping to occur. Upon removal of the large fragments, the clumping disappeared. Right. The front of the same tank. Note the worm tubes extending from the surface of the sediment through the oxygenated layer. Movement of worms in these tubes pumps water into the lower levels preventing them from becoming completely anaerobic, and facilitating the biological filter.
Rapid bacterial growth rates only occur without competition for space or nutrients. As the bacterial populations fill in all the open spaces growth slows and may stop altogether. Some bacteria also secrete a exterior covering called a glycocalyx. These are made of a hard sugar-like material similar in consistency to rock candy. Rapid bacterial growth may produce so enough of this material to glue sediments together. These sediment lumps may be glued so tightly together that hammering is needed to break them apart. In much reef literature, these lumps are said to be caused by calcium carbonate or calcium phosphate precipitation. Such mineral precipitation is rare; if a small sediment lump is placed in a weak solution of household chlorine bleach, it breaks down to the component sediment grains in a short time. If the lumps were formed from the calcium salts, they would not dissociate in the bleach.
