Substrate in the aquarium – the basis for plants and biological balance.
Substrate is perhaps the most important part of planning and equipping a future aquarium. It is an essential component of the biological balance in the aquarium, as it is not only a decorative element; it is needed in the aquarium not just for beauty, but also as a substrate necessary for anchoring plants and as a habitat for most bacteria. Along with the background, lighting, and additional decorative elements, substrate is necessary to give the aquarium beauty, individuality, and in some cases can serve as the basis for aquarium design.
Of course, in fishkeeping, you can often encounter hygienic-type aquariums completely without substrate, equipped only with necessary equipment and, perhaps, with a spawning substrate. An aquarium can be maintained without substrate as well. In this case, all plants are planted in separate pots. The substrate contained in the pots quite ensures the normal existence of fish and plants. The size of these pots depends on the size of the aquarium and the planted vegetation.
Certainly, all such aquariums have a right to exist, but while in most cases—during spawning, raising larvae, and rearing fry—substrate is unnecessary and even harmful, when growing and keeping breeding pairs, many breeders recognize the necessity and usefulness of substrate. In many cases, various specific substrates also serve as a necessary spawning substrate for many fish. And in the decorative aquariums we are interested in, at least in the vast majority of cases, you cannot do without substrate.
Substrate serves as a substrate for plants, and also provides suitable conditions on the surface of its particles for the existence of a complex community of bacteria, fungi, bryozoans, and other microorganisms that process fish waste products. Soil bacteria, protozoan algae, and unicellular living organisms located in the substrate have a significant impact on the biological balance processes in the aquarium. In other words, it works as a primitive mechanical filter into which macroscopic particles and suspended matter polluting the water fall.
Organic and inorganic substances that accumulate in the substrate are broken down by soil bacteria to form salts necessary for the normal development of plants. Thus, substrate serves not only as an aquarium decoration, but also, in combination with plant activity, can be a very efficient tool for filtration and maintaining biological balance in the aquarium.
Any mistake in an aquarium can be corrected, except for the substrate. Substrate can be added, but replacing it means a complete restart of the aquarium, with all the ensuing consequences. Substrate is the most critical stage in setting up an aquarium.
Naturally, in order to work in such a manner, the substrate must be properly selected and properly arranged. Despite its apparent simplicity, aquarium substrate is a complex multicomponent system. At least two main components can be distinguished in it: mineral (skeletal) and organomineral.
By mineral part we mean sand, pebbles, gravel, large stones, i.e., various rocks and minerals, artificial materials (glass, expanded clay, plastic). It is used as a substrate for planting and holding plants, creating decorative aquarium design, and makes up the bulk of the aquarium substrate. The mineral part of the substrate consists of rocks and minerals and can also be divided into parts: • resistant to chemical weathering; • soluble (unstable) in an aqueous environment (carbonates, sulfates, chlorides). As an addition, toxic or dangerous natural compounds (compounds of sulfur, arsenic, antimony, mercury) can be singled out, but unless you are setting up a mineralogical exhibition from aquarium decorations, the probability of such substances entering the aquarium is practically reduced to zero.
In the process of interaction with water, numerous bacteria, and humic acids, a number of minerals and rocks undergo destruction, i.e., physical, chemical, and biological weathering or their combination. Its intensity depends on many parameters, but it is small enough to provide fast-growing plants with all the necessary nutrient components. If the mineral part is not actually a source of nutrients for plants, then it needs to be changed only if, for example, the aquarist gets tired of it.
Over time, the exhaustion of the components of the special substrate containing nutrients will occur, and they will become ineffective. In this case, they will need to be replaced. Most types of nutrient-rich substrates continue to slowly release nutrients for up to 3-5 years. Replace the substrate only if the plants show signs of distress. Well-rooted plants are able to find nutrients even when their release from the substrate decreases. Removing or replacing the substrate can damage well-established plants, so you should act carefully to minimize negative consequences.
To some extent, this also applies to the chemical components of the substrate. Of course, it is unlikely that you, not being a specialist, will be able to determine the mineral composition of the substrate solely by its appearance, and even less likely that you will run to submit samples of the gravel or sand you like to the nearest chemical laboratory to detect calcium salts or heavy metals. Fortunately for us aquarists, we can judge the suitability of the substrate in terms of its chemical composition by a number of indirect signs.
For instance, an abundance of rusty streaks often indicates a high iron content, and consequently, the impossibility of using such substrate in the aquarium (generally avoid shiny crystalline inclusions if possible, as they may turn out to be metal salts, including heavy metals). Green or blue colors may well indicate the presence of copper or chromium. A milky-white color, coupled with slight friability, most likely indicates an excess of limestone inclusions.
Such substrate easily parts with its carbonates, seriously affecting the hydrochemical regime of the aquarium, mainly water hardness, which is extremely undesirable when keeping, say, natives of South American biotopes and at the same time favorable for endemics of African lakes. Carbonates can also be detected by the reaction to adding a few drops of vinegar or hydrochloric acid to the substrate: effervescence, fizzing, formation of a vapor cloud indicate their presence. But even if carbonates themselves are not dangerous for the inhabitants of your aquarium, keep in mind that limestone and dolomite rocks, due to their porosity, are very hygroscopic—they absorb all sorts of filth very quickly and can well turn previously clean water into Mendeleev’s periodic table.
Fortunately, the vast majority of minerals have extremely low solubility in water, and from the point of view of their impact on aquarium water, they can be safely categorized as inert and safe. Nevertheless, if you know for sure that nearby areas are rich in metal ore deposits, try to choose more distant locations for harvesting substrate.
The organomineral part consists of clay minerals, laterite, various organic compounds, waste products of aquarium organisms, soil preparations for enhancing plant growth, etc., i.e., substances involved in plant nutrition. The organomineral part is subdivided into clay (mineral), organic, and bacterial components. It is significantly smaller in volume, but it is precisely here that various chemical and biochemical reactions intensively take place. The decomposition of organic matter and the activity of various bacteria form diverse chemical compounds that can then be easily assimilated by plants.
It is precisely this part of the substrate that most of the efforts of aquarists growing plants and developments of major aquarium companies are directed toward. Suffice it to recall clay balls, peat, various soil tablets, granules, and nutrient mixtures; if any mineral substrate is present in them, for the most part, I think this is done to increase volume. Consequently, the most logical actions in case of a nutrient deficiency would be to increase or change the quality of the organomineral part of the substrate.
It should be noted that full-fledged processing of organic materials, as well as the ability of the substrate ecosystem to self-tune and adequately respond to frequently changing conditions of aquarium life, are possible only with the harmonious coexistence of a wide variety of microorganisms. For this to happen, each of them must occupy its ecological niche in this community, and find suitable conditions for its existence. In our case, this primarily concerns oxygen conditions in the substrate.
A “correct” substrate must allow both air-breathing organisms and anaerobes—which do not tolerate the presence of oxygen and ensure the deepest processing of decomposition products—to live. At the same time, it is important that these conditions remain in balance within the substrate, so that there is a place for everyone and certain microorganisms do not start suppressing others.
In practice, this may look as follows: the upper, thicker layer of substrate, which is in direct contact with the aquarium water and its inhabitants, is well aerated, while the lower, less thick layer remains oxygen-free. In this case, traces of fish (and other) activity, falling down through the thickness of the substrate, undergo primary processing in the upper layers. Part of the products of this processing is assimilated by plants, while another part goes lower, where it falls into the sphere of influence of anaerobes in the oxygen-free layer. They continue and complete the decomposition process. As a result, new compounds are formed, which are partially assimilated by plants again, and partially represent gaseous products and water.
Where to get substrate for the aquarium?
First of all, in nature. The most suitable substrate for an aquarium can be found in river and stream beds. There are also “deposits” of similar gravel: on railway and highway embankments, construction sites, and garages. It is obvious that using substrate from such sources is undesirable. Washing it completely of petroleum products, heavy metal salts, and phenols is most likely impossible. Crushed stone substrate is hardly suitable for the aquarium. It is unattractive, and it is difficult to plant anything in it. Such substrate often injures plant roots as well as digging fish. Gathering substrate on your own is a low-promise endeavor. Of course, in the height of summer heat, washing 2-3 buckets of excellent river pebbles can be a very pleasant pastime, but the remaining 8 months of the year it is absolute hellish labor.
Most hobbyists buy substrate. This can be done in pet stores and at the Bird Market. Any substrate from the Bird Market requires additional processing. In pet stores nowadays, branded substrates are mostly common. They are usually nicely packaged in plastic bags, from 1 to 25 kg per package. In most cases, however, you get a kind of semi-finished product that needs to be worked on to make it suitable for the aquarium. If you make a purchase at the market, try to avoid random sellers, because there are no guarantees that they didn’t dig up their goods somewhere in a drainage ditch.
What substrates exist and are available to hobbyists.
Preparing driftwood for placement in the aquarium. 
