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Aquarium substrates, the gravels and sands of tank bottoms, perform several functions in planted tanks; as root anchors, ballast for weighing down nutritive soil, possibly acting as buffers, mineral sources, even inorganic catalysts! Heck, they also make the system a lot more natural and attractive. Imagine tanks with bare bottoms; no thanks.
Substrates are important; their depth, size, granule shape and composition can make or break an otherwise ideal live plant set-up. Here are my ideas on what to look for and avoid in choosing and using aquarium sand and gravel for the aquatic gardener.
I know it's not the first time, nor am I the originator, but I'll repeat it here: "aquatic plants will grow in almost any medium". Having seen aquarium plant species used around the world in the wild and in aquariums, in mud, muck, dirt, gravel, boulders... most anything more dense than water, I won't dispute that another's suggestions of novel substrate use are invalid.
Nor can a person discount that different plants, cultivars "like" different substrates. So how can we describe which gravel/sand to use where? What we are offering here is a set of generalizations that will/do work for any given set-up and plant mix. Should your tastes turn to biotopic presentations, great; more power to you. Go ahead and investigate, attempt to duplicate that slice of the environment. For now, I'll present my ideas on what a mere mortal can hope to do with what materials are generally available.
Looking around fish stores, garden and home improvement outlets, perusing the scant practical literature, what do you see? A whole lot of possibilities and conflicting opinions on what to do. There are many types, grades of gravels and sands offered and endorsed by various parties... and, yes, their differences really do make a difference.
Composition, or what the substrate is made up of is crucial. You definitely want to avoid any metal contamination (iron can be tested with a magnet), and if the material is soluble at all, you want it to dissolve slowly, in ways that favor your livestock. Overly alkaline matter can be easily "acid-tested" with a drop of muriatic (3M hydrochloric), aka swimming pool or masonry acid, or straight (concentrated) vinegar. Does it fume excessively? You don't want it.
Further notes of real use are put forth below under the Types Available heading. To be concise; you want to avoid sharp silicious matter, ones with deleterious leaching; and seek substrates with angular, slightly basic/alkaline qualities.
Substrate Particle Size:
Size is important, in particular the average size of the particles should ideally be about the same. What does this get you? Some degree of uniformity in your substrate bed, including a more constant diffusion and circulation rate through the depths. What size? Not too fine or too big! See the following table for grades:
Particle Size Grades (Dept. of Agriculture.)
Particle Classification Diameter (millimicrons)
Coarse Sand 200-2000
Fine Sand 20-200
In my opinion coarse gravel, bordering on small rocks, of more than a nominal 1/4" to 3/8" is too darn large for practical use. Your soil will leak out along with useful nutrients, and tender roots will be easily crushed, rather than anchored amongst such "boulders".
On the other end of the size spectrum, can the particle diameter be too little? No, and yes. For most plants/plantings will grow in very fine sand, silt, and clay mixes... but practically speaking this arrangement of tiny particles isn't workable. The bottom will be constantly swept up, clouding the tank; plants will tend to come loose; interstitial spaces will get completely clogged... So, how big should "little" be? Once again, in my humble opinion, in most cases no smaller than coarse sand, 0.5-1 mm nominal.
How do you achieve some sort of uniformity in sizing of substrate particulates? Either through buying it pre-graded or doing it yourself. You can buy or make two sieve-meshed screens that will remove the bigger/smaller pieces; and while you're at it let's mention cleaning. Concurrent with grading you may wash the new substrate, or utilize my favorite method, swishing a batch (about ten pounds) at a time in an aquarium-dedicated plastic bucket with a garden hose, pouring it out and re-rinsing till clear. Wash all substrates, new or used before placing them in your system.
Think of the substrates particles as either flat, chip like or more spherical. Which has more between-grain space? Which is less likely to pack down, channel, clog? The one shaped more like me, roundish!
However the spheroidal substrate you're looking for should not be smooth, but angular; the roughness providing more surface are for chemical-physical mineral interaction, and surface area for microbial habitat.
Depth of Substrate:
Perhaps the most controversial piece of aquarium technology in the latter half of the twentieth century is/was the undergravel filter. Praised for its utility and simplicity; vilified for, among other things, it's destructiveness to live aquarium plants.
The principal issue linking substrate depth with U/G (shorthand for undergravel as in filtration) use is aerobic (oxygen present) versus non (anaerobic or at least oxygen limited) conditions that both may present. How much water circulation between and amongst your soil and substrate is beneficial? "Some" is an incomplete, but not meaningless answer.
Rooted live aquarium plants actually bring oxygen down to and to some extent diffuse it around their root areas. Low to no oxygenated bottoms are more conducive to healthy plant growth... but how about the rest of your livestock? What of the oxygen consuming processes like nitrification, converting ammonia to nitrite to nitrate? Well, what about them? If absolutely critical, isn't there other places for these chemical-physical reactions to take place, expediently?
Of course, you say "In an outside, or inside box, sponge, whatever filter". Hey, good idea. In fact, if the system is in that mythical state called "balanced" you may not need or want to use any sort of purposeful filter at all.
Back to the item at hand, gravel depth. Hmmm, here's a statement: "Depending on substrate particle size, use of soils, plant species mix... and more, a depth can be either too much or too little". Too much is a functional definition, by which I mean that anaerobic conditions produce too much in the way of anoxic by-products, causing stinky troubles for your livestock. Too little depth, and you don't get the mostly anaerobic benefits.
Don't give up on me yet. In practical terms we are talking about a few inches of substrate, oh two to four, five inches. Larger grades may be deeper and still afford minimal circulation to plant roots and diffusion of chemical reactions of purpose to the aquatic gardener.
Other Considerations: Reflectivity, hardness, cost...
Do your plants care what color your gravel is? Maybe it does make a difference in promoting algal growth under leaf surfaces.
Substrate Types Commercially Available:
Quartz, silicate, SiO2 sand, the plain masonry/builder's variety is promoted by some hydrophyte growers, but I don't particularly care for it. Silicates are non-nutritive, unreactive, sharp-sided and tend to pack down. Their best quality is price; typically very cheap. If you're settling on quartz, look (bring a hand lens) for one that is more spheroidal than two-dimensional, and avoid "play sand" grades. You want a coarser, #15 (i.e. fifteen average pieces per linear inch) or lower (that is larger size) number grade.
Other silicates:Crushed glass, marbles. One's too sharp, both are too microscopically smooth.
Flints, are crushed silicates; they are good-looking and cheap in areas of origin, but can be a little sharp for catfishes and other livestock that cut easily.
Volcanic crushed rock, cinders; this igneous rock is unsuitable for the same reasons as all the other silicates mentioned above; it's too sharp edged, and lacks porosity and buffering action.
Coated Gravel, colored or not, pebble or flat pieces, are also inferior in my opinion. Aesthetic considerations aside (maybe you like pink and lime green together!), epoxy, et al. coated gravels lack surface area and possible mineral contribution. Worst case scenario are some off-brands of sealed dolomitious material that aren't quite sealed. Beware these that don't rinse clear on rinse-cleaning.
Calcareous Gravels and Sands Dolomite, Marble, Coral Sand, Marine shells, etc.: are compounds of varying make-up of calcium and magnesium carbonate. Some "soft" dolomites are quite soluble; all calcareous substrates should be used only with plants that prefer or will tolerate hard, alkaline water. These are few.
"Natural Gravels"; are my hands-down favorite choices. The best of these are "river-run" collected; they're blends of hard silicates, including basalt, feldspars, granite and more. The commercially bagged varieties are graded, angular, and alkaline enough to be of near-optimum use for aquatic gardeners.
Achieving passable growth can be accomplished with most any substrate, indeed, some commercial producers do away with solid media entirely. However, if you intend to have a real aquarium garden, you should pay special attention to your substrate.
Free floating plants obviously derive nutrients directly from the water, and this mechanism has been proved for rooted varieties. However, the many beneficial functions of substrates in aquariums cannot be denied; for anchoring, aiding in filtration, acting as stores and catalysts for mineral and organic nutrients... and for looks sand/gravel is a worthwhile addition.
Don't be fooled by it's lifeless appearance, your substrate, with or sans soil addition is a dynamic, complex place of chemical, physical and biological importance. Optimize your aquatic environment by providing gravel/sand of appropriate composition, size, shape and depth.
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