Basic parameters of aquarium substrate.
Any substrate is characterized by many parameters; some of them are crucial for aquarium keeping, while others do not play an important role. The main question when choosing a substrate is what kind of aquarium you are going to use it for. First of all, the substrate in the aquarium must be easily permeable to horizontal and vertical water flows; the substrate must “breathe”. The gaps between its constituent particles must be large enough to ensure good water exchange.
If this condition is met, substrate maintenance becomes very easy. Such a substrate is easily siphoned with any design of siphon, debris easily falls into the substrate and does not cloud the water, but at the same time is just as easily washed out of it during cleaning. Bottom currents in a proper substrate are sufficient to saturate plant roots with oxygen, rotting processes take place painlessly for the aquarium, and no hydrogen sulfide or methane is released.
if you believe that aquarium decoration should follow naturalistic trends, then the color of the substrate should also be close to shades typical of nature: sandy, light and dark brown colors will be quite appropriate here. If you are a supporter of surreal or futuristic plots, then it is preferable to buy a substrate of a bright, pure, saturated color: yellow, white, blue, black, red, etc. Regarding the color of the substrate, muted, dark tones are preferable for a traditional decorative aquarium. Against this background, both the juicy shades of aquatic flora and the bright colors of aquarium fish look more contrasting and advantageous.
This does not mean, of course, that a light substrate has no right to exist, but the formation of an attractive underwater landscape in this case requires a more thoughtful approach, associated with a significant restriction in the choice of fish and plants of suitable colors. The use of substrates of different colors can fundamentally change the color scheme of the aquarium interior; by choosing a substrate of a certain color, you can emphasize the coloration of certain plants or fish. The darker the substrate, the more contrastingly fish and plants stand out against its background. Many fish lose their bright coloration with a light substrate. Light substrate very often consists of rocks that release carbonates and bicarbonates into the water, increasing water hardness and pH.
In general, the color of the substrate can be absolutely anything. The main thing is that the owner likes it. Of course, it would be nice if the plants and fish matched the substrate in color. And if all the internal contents of the aquarium also match its finish, and ideally all this is tied to the room interior or its details, then you can truly be proud of your work and taste. Nevertheless, the final choice is yours.
Really important characteristics of the substrate are shape and size. It is unacceptable to have particles of uneven shape, with chips and sharp edges. For a number of reasons, it is preferable that the substrate particles are not only smooth, but also similar in shape. This is a substrate in which the particle size is approximately the same. They can be three millimeters or eight. This is not very important, the main thing is that the chosen size is observed as much as possible in one aquarium, that is, the substrate must be well sorted into fractions.
The reader can assemble a substrate from stones of different sizes themselves — no matter how bizarre layers it is laid in the aquarium, after the first few cleanings and not without the help of digging fish, such a substrate will come to one result: small particles will fill the gaps between large ones, water in the substrate will begin to stagnate, the substrate will stop “breathing”, swamping will begin, anaerobic rotting of organics with the release of substances toxic to fish and plants, in a word, the aquarium will begin to die before your eyes.
Rounding and the closeness of the shape of the stones to spherical are also very important. Imagine that you are placing some identical objects of different shapes on the bottom of the aquarium. You can always lay out rectangles with almost no gaps, but with circles or ovals, gaps cannot be avoided. Thus, a properly prepared substrate settles by itself as needed, without your tricks.
In a decorative planted aquarium, the most appropriate size would be 4-6 mm. The use of a finer substrate, 2-4 mm, is justified in the case of growing a large number of small foreground plants, such as Hemianthus micranthemoides, Glossostigma diandra, Lilaeopsis, Echinodorus tenellus, etc. On the contrary, for many catfishes and cichlids, a substrate size of 5-8 or 8-10 mm or even larger will be more suitable. However, in such a substrate, many plants grow worse, and some do not take root at all. For large fish, especially those with a habit of digging in the substrate, the bottom can be covered with large pebbles or lined with small boulders.
In addition, for some species, especially those spawning in the substrate, burying themselves in it, or having delicate, easily vulnerable lips (many killifishes, loaches like Acanthophthalmus), only a very soft substrate is suitable — small pieces of peat or peat crumbs. For bottom-dwelling fish, as well as those that bury themselves in the substrate when frightened or in danger, as well as in search of food, the bottom should be covered with coarse sand that does not contain fragments with sharp edges. Small rounded basalt or granite gravel is more suitable for large fish. The smaller the substrate particles, the smaller its layer should be. In addition, the size of the substrate must to a certain extent harmonize with the size of the aquarium, especially regarding small and ultra-small water bodies.
The thickness of the substrate depends on the size of the aquarium and the types of cultivated plants. Usually, the substrate thickness is 3-8 cm. The thickness of the substrate layer is mainly dictated by the need to provide normal conditions for the root system of aquatic plants. For small aquariums, it is usually 3-4 cm, for spacious ones — 5-8 cm. A substrate layer above 7 cm is considered ineffective due to poor permeability and an excess of anaerobic (oxygen-free) zones. A layer of less than 3 cm is ineffective in terms of holding the plant root system. To set up the bottom of a 200-300-liter aquarium, you will need from 2-3 buckets or about 30-40 kg of substrate.
Glow-in-the-dark aquarium substrate.
What substrates exist and are available to hobbyists. 
