Sand Substrate for Plants.
Coarse sand promotes more lush growth of aquatic plants. Fish excrement and other organic residues easily fall between the sand grains deep into the substrate and serve as fertilizer, while plant roots are freely washed by water and breathe easily. As the main component of the substrate, coarse, unwashed sand low in lime is recommended.
An excellent and easily accessible substrate is coarse sand with a particle size of 2-4 mm. It has sufficient porosity. Metabolic processes in such a substrate are not disturbed for a long time. Silted substrate consisting of coarse sand is well suited for most aquarium plants. It suits both plants with a powerful root system and plants with delicate, brittle roots, as its relatively small particles almost do not injure the roots during replanting. This substrate is easily permeable to newly forming roots.
It can be covered with quartz gravel with a maximum grain diameter of 3 mm. Depending on the nutritional needs of the plants, clay can be added to the sand (without 
mixing), as well as laterite soil sold in pet stores (which usually already contains added microelements) to increase nutrient reserves for the plants. Plant two plants in the same aquarium in cleanly washed sand: *Myriophyllum* and *Cryptocoryne*. Within a few days, *Myriophyllum* will begin to grow vigorously, while *Cryptocoryne* will not produce a single leaf. The reason for this phenomenon is clear: *Myriophyllum* takes mineral nutrition directly from the water, while *Cryptocoryne* takes it only with its roots from the substrate. The reason it doesn’t grow at first is because pure sand lacks the salts it needs. However, when using such additives, there is always a danger that they will heavily compact the substrate over time.
One of the most important characteristics of a substrate is its porosity. Fine sand, with a particle size of less than 1 mm, is poorly suited for creating a substrate. It quickly packs down, disrupting metabolic processes. As a rule, the biological equilibrium in aquariums with a substrate consisting of fine sand is very unstable. In fine, tightly packed sand, the roots of aquatic plants develop with difficulty, often rot, and organic turbidity settles only on the surface. If it is not removed in a timely manner, fish will muddy the water, and if removed thoroughly, plants will lack nutrients.
Disruption of gas exchange in such a substrate leads to acidification and rotting of plant roots, releasing hydrogen sulfide, which is toxic to fish. The best substrate in an aquarium is river sand measuring 1.5-4 mm. If the grains are smaller, the sand will pack down, water will not wash the plant roots, the roots will suffocate, and bacteria will have nowhere to settle; if larger, various organic residues will accumulate between them, and gradually humic acids will kill all life in the aquarium. Even fish and mollusks that stir the substrate are of little help here.
Substrate consisting of very fine particles (e.g., fine white sand, including marine sand) is too dense and poorly ventilated by water. Both bacteria and plant roots will suffocate in it. Moreover, forming a multi-level landscape from such material is very difficult: the sand will simply “flow” over time to form an absolutely flat field. Fine sand is usually used in a mixture with other components, such as peat, or as a top layer. Fine sand is not recommended when using an undergravel filter, as it quickly clogs with dirt and there is a risk of anaerobic zones forming. Red or yellow river sand containing iron oxides, which are harmful to fish, must not be used.
Gravel and Pebble Substrate for Plants.
Sand, pebbles, and gravel share a common origin: they are small particles of granite, a rock widespread in our region, making them the most accessible for aquarists. The color of granite particles can vary widely: from light gray to almost black or red. It depends on inclusions of various minerals, but practically any type of granite is suitable for an aquarium. The solubility of various inclusions in granite is very insignificant and does not substantially affect the chemical composition of the aquarium water. Only silicate rocks are the main components of the substrate for plants. Other types of substrates should not be used at all in these situations or can serve only as auxiliary, but by no means mandatory, additives.
As a substrate, it is best to use chemically neutral fine gravel with a fraction of 2-5 mm. There are various opinions regarding the fraction size, but this option is the most versatile. The more homogeneous the substrate fraction and the better rounded it is, the less it packs down, and no stagnant zones are formed in the substrate. Actually, anything can be used as a substrate – from small glass balls to branded polymer-coated sand – the main thing is that it suits you and satisfies the requirement for homogeneity.
Fine gravel (2-5 mm) is usually the main component for preparing an aquarium substrate. It is not a source of nutrients for plants and does not have the ability to retain them. It serves to anchor plant roots, as a top layer over peat, etc. On the other hand, gravel allows nutrients from the water to penetrate inside, where they are absorbed by plant roots. Small pebbles with a particle size of 4 to 8 mm are successfully used in aquariums. Such a substrate packs down less than a substrate consisting of coarse sand, and its silting happens more slowly.
Gravel and pebbles with particles up to 8 mm can be used if the plants have a strong root system. Relatively large substrate particles injure plant roots during replanting, so this substrate is best suited for large plants with a strong root system. Much less frequently, and only in very large aquariums, relatively large pebbles or gravel are used as a substrate. They are practically unsuitable as an independent substrate for growing plants. Large pebbles are used when keeping goldfish, certain species of catfishes, and other substrate-digging fish species.
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 seashells are most indicative in this regard, as they alkalize the water and increase its hardness. The pH of the substrate should be either neutral or slightly acidic (pH=7); this is ideal for most plant roots. Optimal from the perspective of decorative aquarium inhabitants, proven by many years of aquarist practice, is a particle size of 3-8 mm. A favorable microenvironment for the root system of aquatic plants, microflora, *Melanoides* burrowing snails, many species of catfishes, some *Cichlidae*, and other representatives of ichthyofauna whose activity is more or less connected with the bottom forms in a substrate consisting of granules of such a fraction.
Large granules will block root growth, while a substrate with smaller granules will crush these roots by pressure. A substrate with sharp-edged granules destroys roots at the time of planting. The main substrate, if it is insufficiently permeable like unfired clay, blocks the passage of important particles and prevents the penetration of oxygen, which roots need. Any other substrate that meets the above conditions can be safely used in the aquarium, but glass balls, colored ceramics, and other unnatural materials are not recommended for use in the aquarium as they are unesthetic.
Note that when creating the substrate, for each layer, homogeneous fractions are not used—for example, only 3 mm grain size—but rather a mixture of pebbles with a small difference in size. A substrate of a homogeneous fraction of 2-3 mm, previously considered optimal for a planted aquarium, quickly packs down and clogs, becoming completely anaerobic, and does not provide optimal conditions for plant growth and organic processing in the substrate, being poorly permeable to plant roots. A multi-layer substrate should be used.
A mixture of pebbles of different fractions with grain sizes from 3 to 5 mm and from 5 to 8 mm is taken because if pebbles with a greater grain difference are taken, the homogeneity of the mixture is disrupted: they simply do not mix—larger particles will always be on top and smaller ones at the bottom. Of course, as we dig in the substrate, detritus will fall into the lower layers, but still, the aging process of the substrate will be much slower than with coarse pebble fractions. In addition, detritus already processed in the upper layers will fall through, which will not create major problems for plants.
Fine substrate forms an environment with a low redox potential inside itself much faster, retains it much more effectively in the early stages of aquarium existence, and has a milder effect on the water’s redox potential (due to the clogging of the top layer with detritus). Usually, the medium fraction is used in large planted aquariums, and the fine fraction in small aquariums with small *Cryptocoryne* and stem plants. The coarse fraction is good for aquariums with *Cichlidae* and other large fish.
