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Aquarium substrates used to also be known as filter beds; at their best historically a marine "gravel" provided the stabilizing influences for a system chemically, physically and biologically. With advances in filtration, studies in what actually goes on in captive marine waters, and improvement in initial health of livestock, to some, substrates have become an anachronism. Many 'advanced' systems have no sand or gravel at all; instead utilizing live rock, water additives, and/or novel filtration methods to remove organics and buffer water chemistry.
Gravel/sand provides living space for beneficial microbes, alkaline reserve and more for a system. There may be behavioral benefits for your livestock... and, it looks good too. Substrate can fulfill a biological function, but is not absolutely necessary for a successful marine aquarium. Most aquarists and their systems, though, will end up with some form of substrate in them or their filters. Follow along with me on a discussion of the Uses, Properties, Varieties, and Application/Maintenance of substrates.
You will find this is yet another area of the marine interest fraught with mis- and dis-information. Yet, I can promise you, all is not voodoo. By considering what is written here, you'll be empowered to choose whether you want to use a substrate, what for, the type/size/shape/grade/amount, and how to keep it functional.
Uses: Biological, Buffering, Biomineral, Psychological, Looks
1) Bacteria Homes:
First and foremost, gravels, sands and "live rocks" are used in closed systems to support populations of beneficial bacteria. There is a definitive relationship between the microbes and macrobes (livestock) in your aquarium. The most celebrated of these 'bio-geo-chemical cycles' is a portion of the element Nitrogen's. Ergo you will repeat after me, "fishes/macro-invertebrates consume largely proteinaceous foods, the catabolic processes of which result in the excretion of ammonia (NH3, NH4OH), which with the action of Nitrobacter (et al.) bacteria is converted to Nitrites (NO2), then further modified by Nitrosomonas bacteria to Nitrate (NO3) compounds. You definitely want the above chemical reactions to occur at a ready rate; ammonia and nitrites are deadly toxic, and must be diluted, chemically removed and/or biologically processed to low concentrations.
Still other bacteria can anaerobically digest nitrates back to the ubiquitous Nitrogen (N2) gas, which exits the tank to join seventy nine percent of our air environment. Oh, what a happy story, and a valid one.
Well, where do all these bacteria 'live'? Mainly in and around the substrate, though they are found on the tank walls, decor, inhabitants... Do you need them? Strictly speaking, no. But you do have to provide other means to limit or remove the poisoning waste products and by-products of your fishes/invertebrates.
Many/most aquarists opt for the 'gravel route' in their tanks or filters as the least expensive, most secure method of getting rid of metabolites and providing a safety-margin for the eventuality of over-feeding, unnoticed death, or rapid increase in stocking load of their system. By having lots of space for beneficial microbe growth/metabolism bottle-necking of vital chemical and physical reactions is eliminated; balance (in your favor) is preserved.
Fooling with your water with additives, meters, dosers, plastic media, et al. will get you a 'more controllable', though not necessarily controlled environment. Only you can decide for yourself whether the cost in gadgets and human-intervention is worth the loss of automatic stability in utilizing substrates.
Yes, gravel/sand adds to maintenance, cost in a system; and may introduce pollutants... I still use them and encourage you to do so.
You and your fish tank are living proof of the Second Law of Thermodynamics; "All systems tend to a state of greater disorder". Chaos rules, particularly in a small volume of water; everything runs downhill. One measurable way this happens is pH. The above feeding results detailed above result in a drop in the system's capacity to resist a drop in pH, the water "becoming acid". The fancy description for the former property is alkaline reserve; it is the system's sum-total ability to keep it's pH at a certain point. You'll know that natural seawater and good artificial mixes support a pH of about 8.3. There are chemical species dissolved, and precipitated (like sand, gravel, rock) that make up the buffering capacity (alkaline and acidic) of your water. What you want is to keep the pH stable and high, ideally in the low 8's.
There are several ways to do this, or alternatively to do your best to reduce the influences which chip away at your alkaline reserve. Good particulate filtration, chemical filtrants that remove organics, frequent partial water changes which add more reserve and reduce metabolites, adding alkaline solids/solutions, ozonizers, protein skimming, ultraviolet sterilizers... all can and do their part in stabilizing, optimizing water quality, including preserving alkaline (and other) reserves.
By utilizing an appropriate substrate you vastly add to the homeostasis and steady-state capacity of your system. This happens substantially on two fronts: Biologically, you have so much 'life' in the system that other outside force-effects are ameliorated; and chemically, the gravel/sand/rock substrate dissolves, releasing chemical species that counter-act falling pH et alia res.. Is this a good deal? Yes.
3) Psychological Benefits:
Remember the joke, "Do fish feel"? The answer, "Sure, they feel slimy". How much stock do you place in the emotional awareness of 'simple creatures'? There is plenty of scientific and anecdotal evidence that aquatic organisms (including invertebrates) are sentient of physical elements of their captive worlds. For my Senior Report in Ecology at San Diego State U., I did a series of experiments on "Substrate Size Preference" by the crayfish Procambarus clarkii, the incredible/edible crawdad from Louisiana. Basically, I set up three ten gallon tanks, each with three sizes of gravel, alternating first, second, or third in placement. I placed a Procambarus, allowed it to adjust for a while and periodically looked in, recording what grade it seemed to stay over. Varying light, water depth, the size and sex of the individuals tested showed the same result. This species of crayfish displayed a statistical preference for finer gravel.
The point is hopefully made; your livestock can tell if there is gravel in their tanks or not. Many of the species of wrasses, eels, sea cucumbers, urchins and more that live "in" the gravel on the reef, fare poorly without same in aquaria. Would you like to continually live over a shiny, reflective "floor"?
Conscientious wholesalers and aquarists provide at least a glass tray or other "habitat" area of fine gravel/sand for such charges. The role and sheer amount of "interstitial life" in marine environments is scarcely appreciated. Maybe the future will find us with computerized, miniature cameras coursing through our systems, revealing what goes on at the small and between-grain level.
Marine substrates are great appearing. There is a wide range of size, shininess, color to choose from. "Empty" tanks are scrutinized as being funny-looking because they are.
Properties: Cost, Size, Shape, Grading, Surface Area, Quantity, Flow Rate, Composition
"What, another expense?" "Okay, how much?" "Are you joking, for rocks/gravel/sand?" You've come a long way from the sand-box, distance, time and money-wise. Substrates can cost a pretty penny. Be a conscientious consumer as well as marine aquarist and check around. There is a wide array of what's available at different price points.
Buy the best of what you want in the function and looks departments, and be satisfied. This is a one-time purchase (for a couple of years) and can be used beyond it's aquarium life for other interesting projects.
See the comments below concerning physical attributes of the substrate. Don't mix and match different ones in the tank or filter. This leads to "clogging and channeling" due to smaller, odd-shaped pieces fitting between and amongst larger ones.
Of the individual particles is important for two main reasons; maximizing filter bed action and solubility sites. You don't want something too large or small; if too little there will be compaction and too little circulation between the pieces, way big and all the stuff you want to trap just goes through.
For the function of solubility's sake, the smaller the size, the better. Imagine dissolving an Alka-Seltzer (tm) in one piece ("Plop") or crushing it up first; which one would fizz faster? More surface area allows faster dissolution.
For these practical reasons a grade of @ 1.5mm (approximately 1/16") on the smallest size, to @ 5mm (3/16") diameter for the largest is what you want.
The more broadly spherical the pieces of the substrate, the more consistent the flow, and therefore less channeling and packing-down. The complete reverse is found in more flat offerings (like silica sand) that lay-down amongst each other, effectively clogging-off water circulation. My favorite mental demonstration of this principle is the comparison of a bed of poker chips versus one of marbles. Get it? Go with the marble-shapes.
The mesh or 'number' of the granules is an average number of pieces to make up a linear inch; so the larger the number/mesh the greater the size. As per the comments in size above you want a particular size in the tank/filter, and for the size to be uniform. Either buy the product pre-screened or make and screen it yourself. At most, the pieces should be no more than twice/half the sizes of the largest/smallest in the part of the system; for the same channeling/packing reasons.
5) Surface Area
Within realistic limits ought to be maximized to increase solubility and space for bacteria culture. Some company's have manufactured novel materials to have seventy plus percent of their volume missing. One, Siporax (tm) is a sintered-glass from Germany.
What you want to remember is that you want something that's more porous than smooth. One indication of a trade-off of solubility and surface area is that the material is duller vs. shinier; choose the less-reflective.
How much is enough? Is there such a thing as too much? Depending on the size, shape of the substrate, consideration of the volume of circulation, bio-load of the system, esthetics... some writers suggest from a pittance to a few inches. I'll go with somewhere between one and two inches above whatever filter plate. Much less for a bare bottom.
It is imminently important to cover all the surface of a filter, otherwise to ensure some circulation through all the filter bed. Periodic moving of decor, stirring or vacuuming through all areas cuts down on the mal-effects of anaerobiosis. More about this under maintenance.
Here's that time tested rule of toe (why should thumb's get all the attention?), about two gallons a minute per square foot of filter/tank surface area (2 gsfm). There is no practical upside limit, i.e. three gallons per square foot per minute would be even better. For smaller systems you can get this flow rate with large airlifts outfitted with (1 mm bubble) uniform (air)stones, or powerheads. For big to humongous tanks you'll want to employ a fluid-moving pump system, possibly in conjunction with an outside power filter rig. The preference here is to run the circulation "reverse-flow", up-through the filter plates after the water has passed through the particulate and chemical filtrants of the outside filter.
The fact that saltwater is slightly alkaline has already been mentioned. The substrate should provide an excess of carbonate ions to neutralize the organic acids of the livestock. A short passage here re the importance of silicate buffer systems. There are other "cycles" critical to complete replication of a facsimile marine environment; one other is the need for silica (SiO2) (see Plotkin, 1979). There are silicon-based substrates that you could add; and/or chemical adjuncts, but happily neither is generally necessary (or advisable) for any type of marine system. Silicates are nearly worthless as marine gravels; they do nothing to buffer pH, are too two-dimensional to support microbial life, and pack-down too easily. Avoid them.
Make your frequent water changes, feed a variety of foods, and don't worry. The likelihood that your system will suffer a "deficiency syndrome" is highly unlikely, for much of this "trace" material will be added in sufficient quantity in these ways.
Of the most likely marine substrates/gravels you're likely to be offered, there is a direct and positive correlation with suitability, cost and carbonate solubility:
(CaMg(CO3)2 are the least soluble source; they barely fizz when cold hydrochloric acid is test-dripped on them. Dolomite, as the poorest buffer, should only be selected for sparsely populated systems, and only for cost considerations. There has been speculation that it is possibly contaminated with toxic metals (Dewey 1984).
(CaCO3, e.g. limestones, marbles are almost pure calcium carbonate; they are more soluble that the various sources/types of dolomites with their regular calcium and magnesium structure, but less soluble than...
Calcite with magnesium impurities.
Coral sand, crushed oyster and coral rock. These are the most suitable media and buffers.
Size (2-5mm), shape (spherical), grading (all the same), and circulation (2+ gsfm, the more the merrier) have been gone over under properties.
Whatever material you decide on, it must be rinsed prior to being placed in your system. Simply place a portion in a chemically unreactive pan or bucket and run freshwater over it while swishing with your hand. You could also use a sieve but the manual whisking removes dust more thoroughly. Don't fret that the water doesn't rinse perfectly clear, the particulate milkiness will settle out in short order.
Load all the cleaned gravel in at once. See previous submissions on Biological Filtration for notes on moving microbes from established systems on substrates, filter media, water to establish nitrification.
of the gravel, moving the rocks, coral, other decor is encouraged. Develop a routine in concert with your water change schedule to do only a portion of the system at a time. This to preserve your filter beds nutrient conversion capacity, while preventing the negative effects of anaerobic decomposition.
Greenfield (1978) suggests utilizing borrowing marine invertebrates (crabs, shrimp, brittle stars, sea cucumbers, etc.) instead of my proscribed vacuuming to keep the substrate turned, uncoated and "loose". We were fortunate to have a going population of amphipod "shrimp" years ago that we could move around our several service accounts, that worked out quite well for this job. I'd still vacuum when doing water changes.
Other authors also speak of the virtues of intermittent gentle stirring of filter beds to evenly re-distribute mulm (is that a great word or what?), therefore reducing clogging and channeling. Don't do this.
And now, how about some of usual cautionary remarks? Be careful to not turn off the circulation through established filter beds (or any filter media); your aerobic microbes will die back quickly (a matter of minutes)
And filter beds are mutually exclusive. Don't treat your main system period, or any with chemically-reactive substrates; You beneficial microbes will be dust, along with the undesirables (and the vast majority which are innocuous), the media will be contaminated...
"Tramp" metals, such as iron waste on beaches where they are collected may find there way into your system. Test for them with a magnet, if you're suspicious (see rust). Others include zinc, copper, arsenic, lead... are more uncommon and rarely a problem, stay out of solution given proper filtration and a high, stable pH. Most other contaminants; pesticides, herbicides, cleaners, much more commonly originate from the household. Be careful and keep your hands out as much as possible.
Loss of Alkaline Reserve:
Alas, even rocks/gravel don't last forever. Even for the fastidious, the substrate eventually looses too much of it's ability to dissolve in the presence of organic acids. Sometimes you can even tell there is less volume of gravel than when you started.
What to do? Make a plan to remove and replace a third or so at a time every few weeks. This needs to be done usually every two to three years, more for instance if you have three foot long lionfishes in a forty-five gallon hexagonal.
You can determine that your gravel's alkaline reserve is petering out by gauging how often your pH seems to be shifting downwards, using an alkaline-reserve test kit (just like the one's used on spas and swimming pools), or observing the ill-effects on your livestock. I encourage you, do not rely solely on alkaline water adjuncts (additives), like Kalkwasser, sodium carbonate, bicarbonate... A large bed of alkaline gravel gives you much more buffering and a wider margin of safety.
Is the latest and not-so greatest sub-specialty in "reef" keeping. Though this stuff has lots of microbes, it also presence many potential problems. Introduction of pests, parasites and pollutants not the least of them. Unless you're utilizing very little (a few grains thick) of this stuff, be wary of it going anaerobic. Keep your eye on it for dark spots... And on the same note, what about:
Are used to convert nitrates back to gaseous nitrogen for removal from the system. They look like what they are, "air and water-tight boxes" with a water feed in and a water feed out, some sort of "substrate" for anaerobic (without oxygen) bacteria to populate, and sometimes a feeding system for adding a source of carbon (usually "sugar", or ethanol).
Some anaerobic activity occurs in every system without the express use of such devices. By and large the units offered to aquarists are gimmicks that require constant attention, do little to improve net water quality, and way too often lead to poisoning of the system. Flow rates through digesters and carbon feeding are tricky matters. We'll have more to say about them under biological augmentation and filtration topics. They are mentioned here because of the involvement of substrates, and filter beds.
Dewey, Don. 1984. Dolomite: Friend or Foe? FAMA 2/84.
Fenner, Bob. 1997. Biological filtration form marine aquariums. FAMA 7/97.
Greenfield, Dick. 1978. Marine Filter Bed Materials. Pts. 1 & 2, 11,12/78.
Greulich, N. & K. Mirsch. 1990. Siporax: the high performance substrate for biological filters in aquaria.
Lemkemeyer, Jurgen. The Substrate (In the Marine Sector, too...). Today's Aquarium-Aquarium Heute 1/87.
Plotkin, George. 1979. Editorial response to Greenfield's pieces. FAMA 3/79.
Siddall, Scott. 1979. Editorial/Bio-Feedback (to Greenfield's pieces). FAMA 3/79.