Mudder.... The Importance of Deep Sand.
By Ronald L. Shimek, Ph. D.
Initially Published in the March,
2001, Aquarium Fish Magazine
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.
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
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
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
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
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
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.
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.
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
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.
Lump formation is a disaster
for the biological filter. The lumps restrict
water flow and trap organic material where it
can rot. Additionally, lump formation shuts down
the biological filter by covering the bacteria
and preventing them from metabolizing nutrients.
This, in turn, causes the tank nutrient levels
Fortunately, prevention of sediment
clumping and the simultaneous maintenance of optimal
biological filter operation is easily done by
the establishment of a healthy and diverse sediment
dwelling fauna, or "infauna." The infauna,
so-called as the FAUNA lives IN the sediments,
is a very diverse array group of wonder-working
organisms. Unfortunately, they are small, and
are not particularly attractive. Like Rodney Dangerfield,
"They don't get no respect." That is
a pity, as they do most of the work in keeping
any reef tank functional.
The infauna are "the clean-up
crew" and the "reef-janitorial"
staff, and the array found in a successful tank
may be DIVERSE! More than 200 different species
commonly are found living in a mature sand bed.
These include many types of flatworms, round worms,
dozens of species of bristle worms, small snails,
brittle stars, small sea cucumbers, protozoans,
and many types of small crustaceans. The total
populations may be immense. I have done sampling
to measure the abundances found in the 45 gallon
tank I mentioned earlier, and the number of animals
larger than half a mm, or about one fiftieth of
inch, in that tank ranges from 90,000 to 150,000
depending on what part of their population cycle
the various species are in.
harpacticoid copepod, about 1/50th of an inch
long. Barely visible, these small crustaceans
are an important part of the food chains and clean-up
crews in our tanks. They live on and in the sediments.
group of tube-dwelling bristle worms, probably
chaetopterids, in my 60 gallon Stichodactyla
tank. These animals are primarily filter feeders
catching small particles with their paired feeding
head end of a small predatory bristle worm called
a syllid. These probably eat other small worms
and move through the sediments in search of them.
This worm was about 1/10th of an inch long.
What does this diverse and abundant
array of critters do for and in the sand bed?
Well, some will eat excess food, detritus, or
algae. In doing so, they utilize it, and excrete
part of it as waste. In turn, bacteria utilize
that, and thus the infauna help keep the biological
filter going. Additionally, many infaunal animals
burrow ingesting some sediments as they go. They
digest the microorganisms off of them, opening
space for bacteria to grow.
By moving through sediments,
the animals jostle and move the particles. Not
much, just a little tiny bit. It has been estimated
that each day each small organism moves about
10 to 100 cubic millimeter of sediment. Multiplying
this tiny average amount of jostling by the number
of animals in the tank gives the total amount
of disturbance. In my 45 gallon tank, with an
average population of about 100,000 small animals,
from one to ten million cubic millimeters of sediment
is moved each day. Or phrased another way, the
entire tank's sediment volume could be completely
turned over at least once every three to thirty
days. With this amount of jostling and sediment
eating, sediment clumping the sediments will simply
food is eaten and disposed of or recycled as animal
or algal flesh, and that the biological filter
is maintained in the best of condition. And, best
of all you, as the aquarist, didn't have to do
anything. The animals did it all for you. All
you had to do was to sit back, and enjoy a healthy
tank. And, yes, I know it was a dirty job, but
somebody had to do it...
baby fire worm found in the sediment bed of my
45 gallon lagoonal reef tank. Juvenile worms such
as this are commonly produced by spawning adult
worms, and have passed through a relatively long
planktonic larval stage in the tank water. Most
of the babies perish by being eaten by corals
and other suspension feeders.
But this isn't all the good a
sand bed will do for your system! Most of the
infauna live a year or less. However, they grow
rapidly and start reproducing within a few weeks
after they were spawned. Cumulatively, they produce
large amounts of small eggs, sperm, and larvae
that are liberated invisibly into the tank's water.
All the spawned material has the potential of
becoming food for many small-polyped stony corals
as well other filter feeders. It is no coincidence
that, historically, aquarists began to be able
to keep many of these small polyped corals when
they started keeping a sand bed in the aquarium
for the first time.
Making a sand bed is almost too easy. The
most important part of the sand bed is, not surprisingly,
the sand. While earlier I referred to "mud"
and now I refer to "sand," I am not
discussing two different materials. There is no
scientific definition of "mud," however,
those of us befuddled folks who spend part of
our life working with marine sediments have a
naming scale for the parts of the continuum of
particles ranging from the very big ("boulders"
= particles over 25.6 cm, about 10 inches, diameter)
to the very small ("clay" = particles
less than 0.004 mm, about 0.00016 inches). Nowhere
in this scale is there a mention of that most
desirable of substances, "mud." Generally,
what a sediment-studying scientist would refer
to as fine or very fine sands with smidgen of
silt, most normal folks call mud. These are sediments
whose particles generally range from about 1/16th
mm (0.063mm) to about 1/8th mm (0.125 mm).
What's all the fuss about
sediment grain size, anyway?
In all of my discussions about sand beds I
have made a point of specifying one particular
parameter, that of the average size of sediment
particles in the sand bed. Why should this one
factor be so important? The answer simply is that
sediment particle sizes determine the acceptability
of the sediment to the organisms. Perhaps an example
might illustrate this statement better. One of
the common amphipods found along the west coast
of North America is a species called Rhepoxynius
abronius. This small bug has been investigated
in some detail as an organism to use to test the
toxicity of sediments, has been found to prefer
sediments of a specific particle size, 0.113 mm
in diameter. If given a choice, it will move to
and live in sediments of that one specific size,
not sediments 0.110 mm nor sediments of 0.115
mm, but only of that one size. If individuals
are experimentally confined to other sediment
sizes, they neither live as long, nor reproduce
as well, nor tolerate stressful conditions as
do individuals kept at the optimum grain size
Most sediment-dwelling organisms
appear to have similar precise preferences. However,
most will also live at least marginally well in
mixed-sediments with sizes around their optima,
and most sediment particle size optima seem to
be in the range of 0.050 to 0.200. Consequently
I suggest a range averaging about 0.125 as a good
compromise. It isn't specifically the best for
most infaunal species, but it will allow a diversity
of species to live pretty well.
A good sediment
particle size distribution for a sand bed.
Coarser sediments such as gravel
or crushed coral are simply too big. Additionally,
they have the drawback of being sharp edges that
are abrasive to many of the small crustaceans
and worms that must crawl through the sediments.
Finer sediments can pack so tightly together that
they are impervious to most animal movement, creating
a layer that restricts animal and water flow shutting
down the biological filter.
Having to assess sands for particle
sizes would be a daunting task for any hobbyist.
Fortunately, however, several vendors sell bulk
sands in the appropriate size ranges, often marketed
as "sugar fine" or oolitic sands. A
few larger particles in the sediment mix is okay,
but larger sediments should not constitute more
than about 15 percent of the total. Under NO circumstances
should you use crushed coral or coral gravel.
These substrates are too coarse and often too
abrasive for many of the smaller organisms to
The organisms do not care about
the sediment mineral composition, only the particle
sizes and shapes. Most aquarists use the commonly
available aragonitic sands to "provide a
calcium reservoir." Additionally their bright
white color is often aesthetically pleasing. However,
if the system's pH and calcium concentrations
get low enough to dissolve significant amounts
of the sediment there are some very severe problems
and all the sand in the world won't help. Very
successful tanks may be set up utilizing black
lava sand, or fine siliceous sand, as long as
the grains are of the appropriate size. There
is some concern that siliceous sand will fuel
diatom blooms, but such blooms may be controlled
by the appropriate grazers. There is absolutely
no need for any subsurface sand structure such
as a "plenum" or shelf. In fact such
structures will reduce the sediment volume that
is available for the bacteria.
To put a sand bed in an established
tank, remove any gravel or crushed oral, and then
add the new sand, about an inch at a time. Don't
worry overly much about the cloudiness in the
water, reef animals are adapted to this and will
be able to tolerate it easily. In a new tank,
simply put all of the sand you need on the tank's
bottom, add water and place your live rock in
or on the sand. There is no need for any sort
of platform to support the rock. I embed the live
rock a bit into the sediment to give it stability.
After filling the tank with water,
some initial bacterial and infaunal source will
need to be added. This is usually "live sand"
or "LIVE SAND." The former is simply
wet sand that has been in a marine system at some
time. It will have some bacteria in it, but little
else, and I consider it worthless. "LIVE
SAND" will have been collected in a marine
area and shipped directly to you or the pet store
where you may purchase it. It often has a reasonable
array of animals in it, and it is what you need
to give your tank a good start. At least ten percent
of your total tank volume, by weight, should be
LIVE SAND; more is better.
Additionally, there are several
vendors offering "detritivore" or "recharge"
kits having several different types of animals
in them. Kits from different vendors are complementary
rather than competitive. Adding one kit is good,
adding two is really better. You will get a more
diverse system faster with more kits. However,
their cost may be prohibitive. Once the kits are
in, let the system go for at least two weeks without
adding fish to allow the live sand animals to
establish minimal populations. Remember these
are living animals, and will need to be fed.
such as this Eurythoe complanata, are among the
most desireable of the animals added in "detritivore
amphiphods are great detritivore/scavengers and
are also commonly sold in faunal "kits."
snails are very good scavengers and are harmless
to all healthy animals in a tank. They need to
have sand surface to bury into, and should be
added at the rate of two to five per square foot
of sediment surface.
Within a week, you should notice
bubbles in the sediment next to the glass indicating
the sand filter is working, within a couple weeks
small tube traces should be visible in places
in the sediments near the walls, and small bug
populations should be evident. After a two week
wait - and more time is desirable - fish may be
added. UNDER NO CIRCUMSTANCES SHOULD YOU
ADD "SAND-SIFTING" ANIMALS SUCH AS BURROWING
SEA STARS OR SOME GOBIES. These animals
are "sifting" the sediment to eat the
sand critters that you need to have thrive. From
this point, gradually add more animals up to the
More imagined than real problems bedevil keepers
of sand beds. The imagined problems are proposed
by people who are ignorant of the sand bed dynamics.
Among these imaginary problems are accumulations
of hydrogen sulfide and detritus, and the need
for sifting. Hydrogen sulfide will indeed be formed
in the lowermost layers of a deep sand bed. It
will NOT migrate up through the sediments to poison
a tank. Hydrogen sulfide is an amazingly toxic
gas, but that toxicity is exceeded by its pungent
rotten-egg odor. The gas will have an exceptionally
strong odor, and will seem overwhelming at levels
well BELOW toxic amounts. If you can smell this
stuff without it literally taking your breath
away, it won't be at a harmful concentration.
There is no real evidence to indicate that it
may reach toxic levels in a deep sand bed.
Detritus build up in the sediment
is another non-problem. If the sediment fauna
is thriving, there will be a slight build up of
fine detritus while the rest will be processed
by the infauna. The final imaginary problem, the
presumed need for sifting in a healthy sand bed,
simply does not exist. Small organism movements
"sift" the sand sufficiently. Any other
sifting of a healthy bed will cause serious harm.
Sand beds recycle materials and
export many of the excess nutrients in an aquarium.
Some excess nutrients are mobilized by becoming
soluble through metabolic processes and need to
be exported either as harvestable macroalgae or
animals, grown in the main tank or a sump.
The only real problem with a
sand bed is the reduction in diversity as the
bed ages. This is caused by extinction and replacement
problems because the volume of our beds is simply
too small for some species to generate self-sustaining
populations. This is remedied, by purchasing a
detritivore or recharge kit or two every year
or so to give a boost to the fauna.
The installation of a live sand bed is easy,
straight-forward, and inexpensive relative to
almost all other aquarium purchases. Once established,
such a bed will contribute much to the success
of a reef tank by providing a biological filter
with sufficient capacity to for most tanks. Additionally
it will provide food for many of the suspension
feeding animals such as small polyped stony corals.
And, it will do this all with a minimum of care
Ott, F. S. 1986. Amphipod sediment bioassays:
effects on response of methodology, grain size,
organic content, and cadmium. Ph.D. Dissertation.
University of Washington. Seattle, Washington.