What aquarium substrate consists of.
The following basic requirements are imposed on the substrate: • The substrate must not contain, even in very small amounts, substances that undergo decay (e.g., humus); • The structure of the substrate must be porous, with a pore size that, on the one hand, ensures ventilation and does not hinder water penetration into the soil, and on the other hand, allows plant roots to easily penetrate. • The ability to accumulate and efficiently transfer nutrients to plant roots; • Slightly acidic reaction pH <7.0, with no large amount of limestone rocks; • Absence of excess oxygen in the substrate while maintaining the minimum necessary water movement between substrate particles to completely prevent anaerobic (virtually oxygen-free) zones; • Must not pack down or clump, and must not damage the root system;
The substrate serves not only for anchoring plants, but also as a nutrient storage, primarily for plants that, according to their natural lifestyle, are accustomed to obtaining nutrients from the soil, i.e., marsh plants. However, it is also important for other inhabitants of the body of water, as it serves as a habitat for numerous bacteria that absorb fish waste and maintain cleanliness in the aquarium. These are the exact same bacteria and saprophytes that inhabit the filter. Animals that live in the lower layers of the water or rest on the bottom (known as bottom-dwellers) also care about what kind of substrate is in the tank.
The main aquarium substrate must meet several conditions. The first and most important condition is that it must not contain elements that alter the physical and chemical parameters of the water. Corals and shells are the most indicative in this regard, as they alkalize the water and increase its hardness. Marine sand and crushed shells must not be placed on the bottom. This is not a substrate for a freshwater aquarium and can only be used in tanks with saltwater. Otherwise, the hardness in a freshwater tank will always remain high, which is unacceptable for most fish and plants. The pH of the substrate should be either neutral or slightly acidic (pH=7).
From the perspective of decorative aquarium inhabitants, tested by many years of aquarists’ practice, the optimal particle size is 3–7 mm. A substrate consisting of granules of this fraction forms a favorable microclimate for the root system of aquatic plants, microflora, Melanoides burrowing snails, many species of catfish, certain Cichlidae, and other representatives of ichthyofauna whose activity is tied to the bottom to a greater or lesser extent. Any other substrate that meets the above conditions can be safely used in the aquarium, but glass beads, colored ceramics, and other unnatural materials are not recommended for aquarium use as they are unesthetic.
Washed coarse river sand can be used as a substrate. Neutral hard rocks such as basalt, granite, and quartz are recommended for a freshwater aquarium. Substrate particles must not have sharp edges that could injure fish. The substrate can be dark or light brown, yellow, or white, but it must be completely chemically neutral. Light-colored substrates very often consist of rocks that release carbonates and bicarbonates into the water, increasing water hardness and pH.
If coarse sand and river pebbles are used as a substrate, the water in the aquarium will have a more or less constant hardness that will gradually decrease over time. Other materials are also used as aquarium substrates: crushed brick, porcelain, ceramic, rounded glass chips, plastic pellets, coal, and expanded clay. However, there is a concern that if organic matter accumulates in the pores of expanded clay, a sharp release may occur over time, turning it into an organic accumulator.
Beginner aquarists should be warned against using reddish or rusty-colored sand and gravel containing iron oxides, which are harmful to fish and plants, as well as 
material taken from quarries or water bodies polluted with industrial waste. Stones and crushed rock with metallic sheen veins are also unsuitable. There should be no pieces of rust or metal-containing minerals among the gravel grains. All of this poisons the tank inhabitants. Of course, the substrate must not contain petroleum products or agricultural poisons. It is impossible to remove them, so if a specific odor is present, it is better to discard such a substrate. The use of garden soil, as recommended by some authors, is unacceptable in an aquarium. Moreover, this substrate will serve as a nutrient medium for the rapid growth of algae, which are very difficult to combat. Concrete or brick chips and substrates containing iron ions must not be used.
It is believed that the substrate should be chemically neutral and poor. Why? After all, plants practically do not encounter such substrates in nature. The answer is quite simple. From a technical point of view, it is easier to know in advance that there is nothing in the substrate and then add the necessary elements at your discretion, rather than dealing with the complex problem of figuring out what a particular rock actually contains. In most tropical water bodies, the water is soft or very soft. Regarding the mineral composition of pebbles, gravel, and crushed stone used for substrate, neutral hard rocks (granite, quartz, basalt) are generally preferred.
It is undesirable to use marble chips, dolomite, limestone, and seashell fragments as substrate, because the dissolution of the calcium carbonate they contain causes 
an increase in water hardness. Marble often contains a large amount of iron, as evidenced by rust spots on the surface. The prohibition against using marble, marine sand, pebbles, and seashell fragments containing limestone-origin rocks applies only to soft-water aquariums with an acidic or slightly acidic environment. The use of substrate with limestone inclusions in an aquarium with fish requiring high water hardness is fully acceptable.
The effectiveness of the substrate in performing its functions depends on its particle size. Coarse-grained substrate creates a specific structure and suitable interior in the aquarium. However, uneaten food remains get trapped between large particles, polluting the aquarium. If the substrate is too fine, water flow between particles is hampered, and its biological filtration deteriorates. Gravel and pebbles with particles up to 7 mm can be used if plants have a strong root system. Large gravel and pebbles are used when keeping goldfish, certain species of catfish, Cichlidae, and other bottom-digging fish. Peat chips are used when keeping egg-scattering toothcarps. In an aquarium with plants having delicate root systems, coarse-grained sand is preferable.
Substrate composition. Sand, pebbles, and gravel. Mixture of pebbles of various fractions.
Why substrate is needed in an aquarium. The nitrogen cycle in the aquarium. 
