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The Generic Sand Bed System
By Sam Gamble

Sand bed systems are passive filters primarily based on the natural functions of microbial metabolism. The principle of diffusion maintains most of the important pathways. The result is the transforming and cycling of the elements we loosely term nutrients. The surface of the sand bed is an active interface that initially produces the environment of reduced oxygen that is necessary for microbial mediation of compounds like nitrate and sulfate.

Since we are dealing with more benthic ecology, then more types ofmetabolic transformations occur. With the application of a plenum, the many desirable features of the anoxic interface are extended, down through the sand bed system via biogeochemical pathways. The process utilizes energy more efficiently.

Okay we went through all that last time. "Parts is parts". - There have been variations to the original design, but lets keep it generic for the purpose of illustration. Then once the concepts are clearly attached to the parts of the system, you can experiment on your own. And, let me know how it turned out.

Basically there are three elements; water in the aquarium, sand layers of the bed, and the space underneath most often referred to as the plenum. Accessories like strong lights and pumps will appear as we come across their need.

Water:

Standard quality sea salts or natural sea water are acceptable, providing it meets the anti-nutrient and pro trace element qualifications.

Customary parameters:

  • salinity = 30 parts per thousand (+/- 2 parts per thousand)
  • pH = 8.2 (+/- 2)
  • temperature = 76 (+/- 2 degrees F)
  • NO2 = 0
  • NO3 = as close to zero as possible (a subject for future discussions)
  • calcium = 400 mg/l (fluctuations are normal, shoot for a steady average)

    All mineral and trace elements are assumed present until after a specimen load is introduced. Remember this is still a construction site, not a functioning community yet.

    Circulation

    It is very important to have complete circulation. A high volume pump will help assure good circulation, but design of the aquascape and plumbing will be just about as important. The end result must flush waste materials and supply necessities like gaseous element exchange.

    Sand layers

    The geological origin (kind of sand) is important, but not written in stone. It is preferable to have an aragonite sand that is high in calcium carbonate and strontium carbonate. The reasons, we'll clarify later as break-in and specimen load occurs. Also it is important to have a well graded consistency of uniform grain size at roughly 2 mm. Diffusion and bacterial densities benefit by doing so.

    Bulk sand

    The majority of the sand bed is the bulk sand and best to be quarried aragonite sand. This is because it will have no natural organic load to contribute undesired organic content later. Populations of bacteria will colonize the empty niche after the "living sand" topping layer is added. Energy yielding elements diffuse and adsorb into the substrate, helping it come to life.

    Living sand

    The living sand top layer is icing on the cake and the inoculation of starter bacteria. It will segregate and prosper as per the rules of benthic ecology dictate and supplied by elemental parameters by the individual character of your particular aquarium.

    Plenum

    The plenum is much like our basements. A lot of things end up down there in the dark to be eventually recycled. What goes on in the living area above will determine how much is thrown down the stairs. Without the basement we have a tendency to accumulate the junk upstairs. How things get up and down the stairs and what goes on in the dark, are still guesswork. So we'll leave this analogy for now.

    Size

    A good volume is very desirable. A one inch plenum has been a successful standard. Slightly larger plenums are becoming more popular, but there seems to be a cut off point at around two inches deep.

    Light or dark

    Most of the debate on whether to allow light to penetrate the plenum is anecdotal to my knowledge. From my personal experience, a dark plenum is preferred.

    Sealed

    Much of what enters the plenum is by diffusion. Reaction rates of elements are assumed to be by rate limiting factors like availability. Take oxygen for example. If the plenum has direct access to the water above the sand bed via a hole or torn grid,results would have to be logically altered.

    In summary let's start at the bottom and go back to the top adding some general dimensions:

  • a 1-2 inch plenum with no access of light or direct influence of the bulk water. The screen and grid method holds the sand layers above the plenum space.
  • a 3-4 inch bulk sand of virgin aragonite sand of roughly 2 mm uniform grain size.
  • a 1 - 1/2 inch layer of icing (living sand) on top, to kick in the microbes and infauna.
  • water with complete and strong circulation that meets all standard parameters and common sense.

    Add some strong lights, that have bounce per ounce, the quality necessary, and you have a reef tank. Then the real fun starts.