Ask the WWM Crew
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Aquarium substrates or gravel fulfills a few functions for us as aquarists. Gravel/sand provides living space for beneficial microbes, perhaps alkaline reserve and other chemicals for a system. There may be behavioral benefits for your livestock, anchoring for plants... and, it looks good too. The majority of aquarists use some form of substrate in their tanks, though strictly speaking they could leave the bottom bare in most cases. Here is our discussion of the Uses, Properties, Varieties, and Application/Maintenance of substrates for freshwater aquariums.
Uses: Biological, Buffering, Biomineral, Psychological, Looks
1) Bacteria Homes:
First and foremost, gravels, sands and other solid decor in aquariums function 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 the element Nitrogen. <refer folks to that chapter on cycling>
Well, where do all these bacteria 'live'? Mainly in and around the substrate, though they are found on the tank walls, decor, even on the inhabitants... You do need them'¦ to provide means to limit or remove the poisoning waste products and by-products of your livestock.
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 bottlenecking of vital chemical and physical reactions is eliminated; balance (in your favor) is preserved.
2) Buffers: pH and Alkalinity of Water
Metabolism and purposeful feeding results in a decrease in the system's capacity to resist a drop in pH, the water "becoming acid". The fancy description for this property is alkaline reserve; it is the system's sum-total ability to keep its pH at a certain point. All sources of freshwater have a varying alkalinity/alkaline reserve. That is, 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 somewhere within the tolerance of your particular livestock. Some animals and plants do best in more acidic water (below 7.0 in pH) like Discus and South American Tetras, other types of livestock appreciate harder, more alkaline water (with a pH higher than 7.0) like the life from the Great Lakes of East Africa. The particular range of these values by species can be found in books like this one and online sources like fishbase.org.
There are several ways to provide for sufficient 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,... 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., RMF 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 catfishes, spiny eels, cichlids and more that live "in" or above the bottom in the wild fare poorly without substrate in aquaria. Would you like to continually live over a shiny, reflective "floor"?
4) Looks: The Aesthetics of Substrates
Several types of freshwater substrates are great appearing. There is a wide range of size, shininess, and color to choose from. Basic choices are natural or not, the latter often being artificially colored then epoxy coated, and therefore chemically inert. Natural gravels encompass flints, silicates, dolomites/coral sand/shell materials, and carbonaceous rock of various types, even crushed (and hopefully tumbled) glass. Other than consideration as to appearances, natural gravels can be useful adjuncts to maintaining good water quality, slowly dissolving over time.
Properties: Cost, Size, Shape, Grading, Surface Area, Quantity, Flow Rate, & Composition
1) Cost of Acquisition:
"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 here and check around. There is a wide array of what's available at different price points. Often, large quantities (even hundred pound bags) will be your best buy'¦ and there are other than fish store sources for gravels'¦ Often "sand and concrete" supply sources carry substrates of use for aquariums.
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 its aquarium life for other interesting projects. Read through this section thoroughly re issues as to color, size, shape, and composition of the material you'll be using'¦ for the types of livestock you intend. One size/type does not "fit all".
2) Size of Substrate:
Size of the individual particles is important for two main reasons; maximizing filter bed action and for natural gravels, 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. For most systems, practical uses substrates of about 1/8 " diameter are "just right"'¦ ones that are larger call for more volume to produce the same effects, smaller diameters, less volume.
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. Our 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. Sharp edged substrates can cause other troubles with freshwater livestock, cutting into the skin of burrowing fishes, even snipping off the barbels of scavenging catfishes.
4) Consistent Grade:
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 channeling/packing reasons.
5) Surface Area
Within realistic limits ought to be maximized to increase solubility and space for bacteria culture. 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. Most folks go with somewhere between one and two inches above whatever filter plate or a bare bottom. This works out to something like ten pounds of substrate per square foot of bottom. Some writers encourage sloping the gravel bed from front to back/ lower to higher'¦ without employing submerged wood, rocks, other decor for this terracing, you'll find "it all comes out level" in a while.
If you're using undergravel filtration it is imminently important to cover all the surface of the plates about equally 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. We'll mention much more about this under maintenance.
7) Flow-Rate if Using Sub-Gravel Filtration:
As a rule of thumb about two gallons a minute per square foot of filter/tank surface area (2 gsfm) is about right in terms of flow through the filter bed. 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. Reverse flow set-ups provide much better for removal of sediment that otherwise collects under filter plates.
8) Buffering: For Those Who Need/Want It:
Your source water may be slightly to greatly alkaline to acidic. The substrate could provide a needed or excess of carbonate ions to neutralize the organic acids of the livestock. You will want to test your water and perhaps pre-prepare it outside of the system in some cases'¦ storing it for future use in a convenient container. If you're fortunate to have tapwater that is "just about right" straight out, it might be reasonable to use a substrate that will aid your efforts in sustaining pH'¦ one that is suitably alkaline and soluble.
Of the most likely substrates/gravels you're likely to find, there is a direct and positive correlation with suitability, cost and carbonate solubility. Once again, you need to investigate 1) Your water source pH and alkalinity, 2) The needs, desired range of these qualities for your intended livestock, and 3) Formulate a plan for providing a stable means of supplying water quality within these values. For people with either overly soft/acidic water or ones who need more alkalinity and higher pH there are gravels that can help elevate both of these.
These composites of calcium and magnesium carbonates (CaMg(CO3)2 are the least soluble source and generally will not raise pH's higher than the 7's. Dolomites come in a few formats, and need to be thoroughly rinsed before adding to a system.
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 shell and coral rock. These are the most suitable media and buffers and of most use for very hard, alkaline water systems like those for Lake Malawi and Tanganyika and brackish systems.
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 however, slight particulate milkiness will settle out in short order.
Load all the cleaned gravel at once if possible. Please refer to the sections on Biological Filtration for notes on moving microbes from established systems on substrates, filter media, water to establish nitrification.
Maintenance: Periodic vacuuming
Vacuuming of the gravel, moving the rocks, coral, other decor is encouraged. It's best here to develop a routine in concert with your water change schedule and do only a portion of the system at a time. This partial vacuuming preserves your filter beds nutrient conversion capacity, while preventing the negative effects of anaerobic decomposition.
Some authors suggest little or no disturbance to established filter beds, some preferring the use of gravel stirring scavengers. I'd still vacuum when doing water changes.
Most aquarium "remedies" and living filter beds are mutually exclusive. Don't treat your main system period if you can avoid it, instead using a hospital tank. By using harsh chemicals in your main/display system your beneficial microbes will be impugned along with the undesirable ones (and the vast majority which are innocuous), and the substrate media will be contaminated...
"Tramp" metals, such as iron waste where substrates are collected or mined 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 uncommon and rarely a problem. Most other contaminants; pesticides, herbicides, cleaners, much more commonly originate from the household or "dirty" hands in the tank.
Loss of Alkaline Reserve:
Alas, even rocks/gravel don't last forever. Even for the fastidious, the substrate eventually loses 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 a crowded, over-fed system.
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. A refreshed bed of alkaline gravel gives you more buffering and a wider margin of pH-drop safety.
About Collecting Your Own Substrate:
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 a well-known trusted or tested source, we encourage you to buy processed substrates.
Anaerobic Digestion w/ Deep Sand Beds (DSB's):
The use of deeper and or finer grades of substrate are used to anaerobically ("without oxygen") convert nitrates back to gaseous nitrogen for removal from the system. There are definite benefits and dangers in these approaches versus the use of live plants, water changes, and chemical filtrants'¦ to alleviate nitrate accumulation. The potential downsides of this anaerobiosis are production of noxious by-products like hydrogen sulfide gas (rotten egg smell), which can be deadly. By and large aquarists should be wary of "bubble" accumulation within their gravel, vacuuming such areas if they appear.
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.
Anderson, Frank G. 1996. All washed up? What your gravel does for you in the aquarium is not only affected by the size of the material, but also by its physical composition. TFH 4/96.
Hawthorne, Harold. 1979. For beginners: Aquarium substrates. TFH 4/79.
Mack, Walter N. and Elizabeth A. Leistikow. 1996. Sands of the world. Scientific American. 8/96.
Mortenson, Jim. 1995. If I had only known (column, on gravel). FAMA 3/95.
Nicholson, Danny. 1978. Don't sweep the dirt under the rug. It's worth the little extra bit of effort to size and grade your own substrate material'¦ It's a lot less work than tearing down your tank to get the silt out from underneath the subgravel filter. FAMA 1/78.
Schiff, Steven J. 1993. Aquarium set-up. Gravel. FAMA 7/93.
Wickham, Mike. 2002. Aquarium substrates. Get to the bottom of things. AFM 12/02.