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Related FAQs: Marine
Substrates, Marine Substrates 2, Marine Substrates 3, Marine Substrates 4, Marine Substrates 5, Marine Substrates 6, Marine Substrates 7, Marine Substrates 8, Marine Substrates 9, Deep Sand Beds, Refugium
Substrates/DSBs, Live Sand, Mud Filtration 1, Plenums, Nitrates, Sand
Sifters, Aquascaping, Calcium, FAQs 1, By Type: Aragonite/s, Coral Sands, Silicates, Dolomites/TapAShell, Southdown & Such, Collecting Your Own, & Physical Make-up, Size/Grade, Location, Depth, Marine
Substrate Cleaning 1, Marine
Substrate Cleaning 2, Moving/Replacing/Adding To, Marine Substrate Moving/Replacing/Adding To
2, Substrate
Anomalies/Trouble-Fixing,
Related Articles: Marine System Substrates (Gravels,
Sands) by Bob Fenner, Marine System
Components, Deep Sand Beds, Plenums, Live Rock,
Biofiltration,
Denitrification, Live Sand, Sand
Sifters,
Choosing a Substrate
for Your Marine Aquarium
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Sara Mavinkurve |
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One of the most important decisions marine
aquarists make before setting up a new aquarium is what substrate to
use (or if they will use substrate at all). This article is an
introduction and overview of the most popular choices aquarists make,
the pros and cons of each, and what you should think about before
deciding what you want for your system.
Points to Consider
The subject of substrate choice has been met with
a lot of heated debate amongst marine aquarists, causing many aquarists
to feel very strongly about deep sand beds (DSB) verses bare bottoms
(BB). Friendships have been tested, hearts broken, cyber
wars waged! Well, not to get too melodramatic, but it's been
a hot topic to say the least. Before you get caught
up in the 'politics' of the matter, think about what you
personally want. What looks good to you? What purpose do
you want your substrate to serve? Will it be a habitat for
life? Will it be an additional source of filtration? Will
it server an aesthetic purpose? Then there are the practical
questions. Do you want something easy and low maintenance or do
you enjoy spending a lot of time with your aquarium? How big is
the system you are planning? What kind of animals do you plan to
keep? One should consider all these questions before deciding on
a substrate. As you'll read below, each choice has its
advantages and disadvantages.
But before I begin this discussion of pros and
cons of different substrates, I want to point out that, in my
opinion/observations, there are three basic types of points that tend
to influence the decisions we make and opinions we form about our
aquariums and the world in general. There's experimental
evidence, anecdotal (or empirical) evidence and theory.
Aquarium keeping can be extremely frustrating in that what we might
expect to happen theoretically doesn't seem to happen actually or
experimentally (and vice versa). Then there are
always those anecdotal experiences we have as aquarists for which we
have no explanation at all and appear to happen only to us.
Thoroughly discussing all points made in science, theory and/or
anecdote on marine aquarium substrate choice could easily fill a
book. Since I'm limited here to a few thousand words, I can
only give you a cursory review of these topics and hope that by the end
of it I've sparked your interest and inspired you to do your own
further research and reading.
Bare Bottom
Bare Bottom
Advantages:
We'll start with the simplest choice: no
substrate. There are a lot of potential advantages to a bare
bottom tank. Firstly, they are low maintenance in comparison to
substrate containing systems. It's always convenient to be
able to simply siphon up debris with every water change.
Secondly, for reef or coral containing systems, it's easier to get
intense water flow without having to worry about 'sand storms'
or piles of substrate gathering in one corner of the tank.
Corals can suffer significantly from sediment damage. This
happens in aquariums when sand lands on or rubs against the delicate
soft tissue of any coral. Not having to worry about this problem
alone is incentive enough for many reef aquarists to forego the use of
substrate altogether (especially for many SPS coral keepers who've
been dying to try out a 5HP Pacer SE2JL HYC supplied surge device if
only they could get the building permit).
Bare Bottom
Disadvantages:
The disadvantage of a bare bottom system is that
you won't have the additional niches for life that substrates add
to a marine aquarium. You won't be able to keep any animals
that require a substrate to bury and/or borrow in. In short,
anything that you might gain from having a sand bed (or other
substrate), you won't have in a bare bottom tank.
Aesthetically, some people find bare bottom tanks visually
unappealing. This writer disagrees with this last
contention. There are some fun things you can do with a bare
bottom tank to make it just as visually exciting (or even more so) as
any tank with a sand bed. Use of starboard is one option.
An even more intriguing look is to grow soft corals along the
bottom. A 'carpet' of star polyps, xenia, and/or
zoanthids can look very cool at the bottom of a reef tank.
Having a hefty population of soft coral polyps along the bottom of the
tank might even provide some help with the 'clean up' of
uneaten food and fish waste.
Sand
There are several different ways to use sand as a
marine aquarium substrate, but I'll focus on the few most common
ways.
Deep Sand Bed
Defining
'DSB'
Before we can discuss deep sand beds, we have to
define the term. Though the very definition of 'Deep Sand
Bed' can be a hotly debated issue in and of itself, I'm going
to give it what I perceive as the most common and practical
definition. Commonly understood, a DSB is any layer of sand three
or more inches deep, with a grain size no larger than 'sugar
fine.' If the grains of sand are much larger than granules of
sugar, you're starting to get into 'crushed coral' or
'crushed aragonite' substrates that don't quite function
the same way as finer grained substrates.
The Nitrogen Cycle Refresher
Course
In essence, the Nitrogen Cycle is the
infinitely complicated path, set of paths, overlapping and
diverging paths, twisted and sinister paths, etc. by which
Nitrogen circles around and through the earth, air and
water. Thank goodness we don't need to know all
that much about it. I painfully recall, and perhaps
some readers of this article do too, being required to memorize
the basics of the Nitrogen Cycle in school. I can
also remember rolling my eyes thinking 'when am I ever going
to need to know this?' Huh, well, believe it or
not, here it is again. The breakdown of nitrogenous wastes
(largely from animals), occurs via the Nitrogen Cycle. This
is not unique to aquariums. I'm sure any
self-respecting farmer could tell you just as much or more about
the in-soil manifestation of the Nitrogen Cycle as any DSB-loving
reef aquarist could tell you about the in-aquarium manifestation
of this process. Grossly over-simplified,
it's the pathways by which:
1) Ammonia is fixed (by
nitrogen fixing bacteria) to make Ammonium.
2) Ammonium is converted
into nitrogen oxides (nitrites and nitrates) by nitrifying
bacteria.
3) Nitrogen oxides are
converted into nitrogen gas and water.
Aerobic bacteria accomplish the first two
steps. Anaerobic bacteria accomplish the third.
Anaerobic bacteria absolutely need an oxygen depleted space in
order to live and do their thing. In a DSB, the goal is to
have the top layer be aerobic (containing oxygen), with oxygen
concentration gradually declining to zero down into the sand
bed. In this way, a DSB can become an excellent
natural filter. Essentially, all biological aquarium
filters work this way (with an aerobic zone in juxtaposition to
an anaerobic zone). However, in all such filters, the
biological filtration capacity is limited by the surface area
available for bacterial colonization. In a DSB, this
surface area is truly immense. If you do the math, the
surface area can get up to an acre or more in the average
DSB!
Of course, as with any over-simplification
of a biological process, I'm leaving out countless side
reactions and other by products. For example, hydrogen
sulfide is a significant by product of denitrification which is
toxic to fish if it doesn't exit the system in the form of
gas (which it is at room temperature). When hydrogen
sulfide precipitates iron sulfides, this is what turns areas of
sand substrate black. This phenomenon is why some zealous
bare bottom advocates call DSBs 'ticking time
bombs.' According to the ticking time bomb theory, the
anaerobic layer of sand in a DSB accumulates toxins and will
eventually 'erupt' in a sudden cataclysmic release of a
myriad of denitrification by products (such as hydrogen sulfide),
causing a total tank 'crash' (sudden death of all live
stock). While I suppose it's possible for such a
thing to happen, I doubt that it often does. For one
thing, as mentioned, hydrogen sulfide is a gas. Thus, so
long as the DSB is well populated with benthic organisms and the
aquarium system employs good circulation and gas exchange,
hydrogen sulfide should bubble out along with nitrogen gas and
carbon dioxide, etc. More poignant for myself personally, I
had an experience with one of my own systems that caused me to
seriously question the DSB time bomb idea.
ANECDOTE: The following is a mere anecdote
and should not be taken more or less seriously than any other
anecdote. I once had the good fortune of watching my 65g
aquarium start to leak along the bottom rim. It started as
a drip, then a steady stream. Well, you know how this
goes. In less than an hour, I was forced to drain the tank
(water, sand and all). I'm embarrassed to admit that I
was terribly unprepared for such an undertaking and did not have
nearly enough pre-mixed water or anywhere for the livestock to go
except a few empty aquariums of various sizes and plenty of spare
Maxi Jet power heads. When I got down to my DSB
(about 4 inches deep), I started to scoop it out in 20oz
slushy-cup-fulls at a time. The rotten egg smell was
room-filling and noxious. The sand just an inch or two
below the surface was black. There was every sign that at
least parts of my DSB had turned into an all-out hydrogen sulfide
toxic waste dump. How long it had been this way, I
don't know. Regardless, all my fish, corals and
invertebrates had no choice but to sit in that same water and
sand for at least 3 days before I could replace the leak
tank. And yet, nothing died. Of course, this is, as
forewarned, a mere anecdote. While I was happy to see
that my sand bed had, obviously, a significant anaerobic zone, I
was not happy to see so much evidence of hydrogen sulfide (and
iron sulfide). This is not ideal. Thus, having
learned of what was going on in my sand bed, I made a concerted
effort to obtain more benthic organisms (worms,
micro-crustaceans, seed shrimp, etc.) which would provide the
necessary subtle movement in the sand to allow hydrogen sulfide
to more readily exit the sand bed. So while I'll
concede that a poorly maintained sand bed with insufficient
benthic life can result in significant hydrogen sulfide
production, I'm not ready to concede that DSBs (even at their
worst) are 'ticking time bombs' that will (or could)
crash your tank and kill your animals at any time.
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The Significance of Grain Size,
Grain Composition and Sand Bed Dimensions
Grain Size:
Arguably, even sugar-fine is likely too
large a grain size to get the full functionality of a sand bed as
a source of nitrogenous waste filtration. The
reason for this is, the functionality of a sand bed as a
biological filter is dependent on the existence of an anaerobic
zone where anaerobic, denitrifying bacteria can carry out
denitrification (in large part, the conversion of nitrogen oxides
into nitrogen gas and water). The coarser the sand,
the less anaerobic space there will be. To understand this
concept, ask yourself; 'would I rather be buried under
marbles or mud?' Buried under marbles, you might
stand a chance of survival since all the spaces between the
marbles allow at least some air to get through. Buried
under mud, you'd suffocate in a minute or so (depending on
how long you could hold your breath). By the same
general principle, a sand bed of coarse sand is going to be more
oxygenated all the way through even if left undisturbed.
With a sand bed of sugar fine sand (i.e. a grain size of ~0.5mm),
at the bottom of 4 inches, you'll likely get at least some
anaerobic zones, but not nearly as much as with finer sand.
I'll note here that this might be the actual advantage of
some 'mud' products sold as substrates for marine
aquariums. Marine mud is, after all, just very fine
sand/substrate.
All this said about the theoretical
importance of grain size for creating anaerobic environments, I
must tell you that some denitrification will
occur even if you use marbles for substrate. But
there's even more to this issue of grain size. The
truth is that a lot of benthic (sand dwelling) organisms are very
picky about the range of sediment grain size they will
tolerate. Some of them are picky to an extreme such
that they will not reproduce or live a normal life span if stuck
in sediment of a grain size even just 0.01mm outside of their
preference. Some organisms will just refuse to live in
finer or coarser substrate. And unfortunately,
you just can't make everybody happy. Dr. Shimek opines
that a sand grain size of 0.125mm is likely a good middle ground
compromise for enough benthic organisms. Alternatively,
some equally intelligent aquarists recommend using substrate of
mixed grain sizes, with grains occurring across a range of 0.05mm
to 0.5mm in size. For those who wish to know more about
this topic of sand grain size and the functionality of sand beds,
I refer you to many articles on the subject written by Dr. Ron
Shimek, Dr. Robert Toonen and others.
In short, the thought is that finer the
sand, the more effective the sand bed will be as a processor of
nitrogenous waste. Keep in mind though, that, no matter how fine
or coarse your sand might be, the filtration capacity of your
sand bed will not appreciably increase beyond a depth of about 5
or 6 inches. In fact, there's likely not much to be
gained by having a sand bed deeper than even four
inches.
Sand Bed
Dimensions:
Generally speaking, the dimensions of the
'foot print' (length x width--not depth) of a DSB should
be at least ~4.5 square feet (or roughly the footprint of a 40g
breeder). Unfortunately, this recommendation is routinely
given and taken without much explanation. Generally,
it's thought that this is the threshold dimension needed to
maintain a healthy population of benthic organisms.
Empirically, for whatever reason, it looks like the filtration
and other benefits of a DSB aren't so apparent when the
dimensions are much smaller than this. Also, again, if you want
to make your sand bed bigger, make it longer and/or wider (rather
than deeper).
Grain
Composition:
When it comes to substrate composition,
marine aquarists often find themselves choosing between aragonite
and silica. Though aragonite is usually favored, marine
substrate does not necessarily have to be aragonite.
There's reason to believe, theoretically, experimentally and
anecdotally, that aragonite provides *localized* alkalinity
benefits which might be advantageous for any number of
reasons. For example, it might aid in phosphate
precipitation and/or dinitrification. However, any
potential contribution (if any) it makes to the overall
alkalinity and/or pH balance of the system as a whole is not
likely significant. Even with a sand bed of fine aragonite,
you will still need to balance your system's calcium and
alkalinity by some other means. If you want to use silica
sand (or 'quartz sand') and you're worried about
soluble silicates, I would advise rinsing the sand *a
lot.* While quartz (SiO2) itself will not add
silicates to your water (since pure quartz is virtually insoluble
in water), you might be hard pressed to find 100% pure quartz
sand. Less than pure quartz sand will inevitably have
some contaminates of soluble silica compounds.
However, it should be easy enough to rinse away any contaminate
that's water soluble.
Now, if I could put a side
bar within a side bar, I'd discuss different commercial
sources for substrate of different grain sizes and
compositions. Instead I'll just note here that
there are places outside of your local aquarium store or pet
store where you can find cheap substrate suitable for marine
aquariums. In addition to browsing your local
aquarium store's substrate selection, don't hesitate to
venture into a hardware store and/or construction supply
store. However, do be sure you know what you're
getting and what you're putting in your aquarium (potential
contaminates and all).
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Advantages to DSBs:
If you have the time and inclination to properly
set up and maintain a deep sand bed, this choice of substrate can have
a lot of advantages. Firstly, as discussed previously, it can act
as an additional source of biological filtration. Again, when set
up 'correctly,' a DSB will have two zones; an upper aerobic
(oxygenated) zone and a lower (oxygen depleted) anaerobic zone.
I'll note here that there are at least a few different theories and
methods for setting up a DSB which focus on the deliberate construction
of these layers. The Jaubert System is one such method.
The Jaubert System
Though arguably not a 'DSB' under my
previously given definition, I'll discuss it here since
it's closer to a DSB than it is to any other substrate
choice. The Jaubert system was developed by French
marine biologist, Professor Jean Jaubert, in the late 1980s, and
became popular in early 1990s. In brief, this is a
system with a layer of sand on top of a layer of gravel that is
suspended over a thin layer of empty space. This empty
space at the bottom of the aquarium, underneath the gravel, is
called the 'plenum.' In the Jaubert system, two
different types of substrate are used to create the
aforementioned aerobic and anaerobic zones/layers.
Additionally, there's this 'plenum' underneath it
all. Therefore, there is water both below and above the
substrate. Supposedly and apparently, setting up the
substrate in this way reduces hydrogen sulfide production and
according to some aquarists, reduces nitrate levels.
Though I've never attempted to use this method myself, I (and
others) have enjoyed theorizing as to what might be going on in
these systems. One such theory is that water moves
through the layers by way of convection which results from a
difference in temperature of the water above and below the
substrate layers. If the water on one side of the substrate
layers is warmer or cooler than the plenum, convection will
create a very subtle, steady 'flow' through the substrate
layers. This flow could possibly create the gas exchange
needed to allow hydrogen sulfide, when/if produced, to exit the
system more easily. Or, if the gas exchange is
significant enough, it might decrease the size and number of
available anaerobic areas in which hydrogen sulfide production
could occur. However, this writer wonders, if the Jaubert
system creates fewer anaerobic areas, how could it accomplish
just as much denitrification (which also requires an anaerobic
environment)? Perhaps the reactions resulting in hydrogen
sulfide production are more sensitive to the presence of oxygen
than are the desired denitrification reactions. If this is
the case, then having a hypoxic environment that's not quite
completely anaerobic might allow for all the same nitrate
reduction while limiting hydrogen sulfide production.
However, I dare not delve too far into any detailed
discussion of Jaurbert systems as this has already been done by
writers far more experienced with them than myself.
There has even been some dabbling into controlled experiments
aimed at comparing the Jaubert system to 'traditional'
DSBs and other methods (see list of references at conclusion of
this article). Thus, I refer anyone wishing to know more
about Jaubert systems to articles written by Dr. Toonen and
others.
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In addition to biological
filtration, DSBs create a whole other world of niches and
micro-environments for marine invertebrates. Many aquarists
believe that these organisms are not only fun to watch, but also supply
food (either as themselves or with their larvae) to corals and other
ornamental marine animals.
Disadvantages:
DSBs require maintenance in that, in order to
function properly, they need to have sufficient populations of benthic
life (both seen and unseen). This can be accomplished in several
different ways. The best, cheapest and easiest way is to simply
swipe some sand from the healthy, well-populated DSB of a fellow
aquarist. A measuring cup or two should be enough to get things
going in your own sand. The next best way is to order live sand online
or see if your LFS sells live sand or 'seeder sand.'
Unfortunately, just doing this once is not enough. Over time, the
diversity of your sand bed is sure to decline as some organisms out
compete others into 'extinction' from your aquarium.
Thus, it's very important to occasionally repopulate your sand
bed. If not properly maintained, a DSB will become at best
useless and at worst, clumps of 'cement' interspersed with
black, iron sulfide coated sand.
WARNING: Do *not* put any carnivorous echinoderms
in your DSB. These include the commonly sold 'sand sifting
stars' which people mistakenly believe benefit the sand bed by
stirring the sand. To the contrary, these star fish are rapacious
consumers of micro crustaceans and other benthic
organisms.
The other disadvantage of DSBs is that it can be
challenging to achieve a high level of water flow without creating
'sand storms.' As previously mentioned, these sand storms
can be detrimental to corals. Also, the sand bed needs to be
undisturbed by any such blunt forces (otherwise, the anaerobic zone
will not form).
Shallow Sand Bed
This will be brief. Shallow sand beds (sand
beds < three inches in depth) have very few advantages. They
might add something aesthetically to a tank. They might also add
some additional niches for life. They likely provide some
marginal biological filtration and denitrification. However, the
disadvantages will almost always outweigh the minute advantages
(especially in a reef tank where you'll have to contend with
'fly away' sand storms). You can't easily vacuum a
shallow sand bed during water changes. And without the organisms
of a DSB to process debris, the substrate essentially becomes a trap
for fish wash, uneaten food, etc. Thus, by and large, in my
opinion, a shallow sand bed is almost always a poor choice unless
it's a mere 1/4in of sand used for aesthetic purposes in a nano
tank less than 15g. While shallow sand beds have all the
same disadvantages as DSBs, shallow sand beds don't have the depth
necessary to harbor the life necessarily to make it a significant
source of filtration or place for debris to be processed.
If you're going to go with sand and if your aquarium is 40g or
larger, I say add the extra inch or two and make it a DSB.
Even if the aquarium is <40g, you might consider a remote DSB in a
larger refugium rather than have a shallow sand bed in the display.
Crushed
Coral
Unlike sand, crushed coral (and similar
gravel-sized substrates) can be vacuumed during water changes.
Consequently, it's relatively low maintenance. However, this
feature of crushed coral becomes less of an advantage and more of a
burden the larger the aquarium size. In large aquariums, it
could take hours to vacuum all the crushed coral. Vacuuming also
disrupts any potential the substrate might have to create niches for
micro marine life. In siphoning out debris, you're going to
siphon out a lot of the worms and micro crustaceans as well. For
these reasons, crushed coral is probably most advantageous when used in
fish-only systems under 50g.
Remote Deep Sand Beds (Best of All
Worlds?)
Remote DSBs are DSBs that are not in the display
tank, but instead in an attached refugium, sump or other vessel.
Just about any water-holding container will suffice. The nice
thing about remote DSBs is that, by keeping the DSB out of the display,
it's easier to have strong water flow in the main/display
tank. A remote DSB is also less likely to be disturbed by fish or
yourself. The only notable potential functional
downside is that some aquarists aren't sure if a remote DSB has the
same potential to provide food to corals as does a DSB in closer
proximity to the animals to be fed. Otherwise, if yourself
torn between sand verses no sand, I highly recommend a well lit remote
DSB/refugium. Personally, as someone who loves marine
invertebrates, I like to use the remote sand bed in a refugium as a
second display. If you add a light (as you probably should)
you can grow macro algae above the sand bed for even more nutrient
export.
Conclusion
At the conclusion of this article, I hope I've
at least provided you with the questions you need to ask yourself
before choosing an aquarium substrate. As with any decision we
must make in life, the most important thing to know is yourself; your
abilities, desires, resources, etc. Once you're familiar with
these and all the basic information provided here, the right choice for
you and your system should be easy (or at least easier) to make.
Some references and further
reading:
Toonen, R.J. 1998-99. What exactly is a
'sandbed,' anyway? A brief introduction to the ecology of
marine sediments, Parts 1-3. Journal of Maquaculture 6(3):42-48,
6(4):62-79, 7:2-9.
Toonen, Rob & Chris Wee (2005) Feature
Article: An Experimental Comparison of Sandbed and Plenum-Based
Systems: Part 2: Live Animal Experiments. Advanced Aquarists Online
Magazine
Toonen, Rob & Chris Wee (2005) Feature
Article: An Experimental Comparison of Sandbed and Plenum-Based
Systems. Part 1: Controlled lab dosing experiments. Advanced
Aquarists Online Magazine
Shimek, Ronald L (2003) How Sands REALLY Work,
Reefkeeping Online Magazine
Shimek, Ronald L (2001) Dearest Mudder....
The Importance of Deep Sand, Aquarium Fish Magazine
Toonen, Rob (2000) Are Plenums Obsolete? Another
Viewpoint Part 2. Freshwater and Marine Aquarium (FAMA)
Magazine, Vol. 23(2), pp. 44-70.
Toonen, Rob (2000) Are Plenums Obsolete? Another
Viewpoint Part 1. Freshwater and Marine Aquarium (FAMA)
Magazine, Vol. 23(1), pp. 44-66.
Sprung, Julian (2002) Jaubert's Method, the
"Monaco System," Advanced Aquarists Online
Magazine
And so many others'¦
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