Gravel is an extremely popular substrate for a freshwater aquarium. Besides sand and soil, it is one of the most frequently used substrates in the aquarium hobby. We explain the characteristics of gravel and how and why it can be used in an aquarium.

Grain size

In principle, the transitions between the products sand and gravel are very fluid. According to the DIN standard, the distinction depends very much on the grain size of the individual particles. Basically, gravel exists at a grain size of 2 to 63 mm, with various subdivisions into coarse gravel, fine gravel and medium gravel. By definition, a grain size of less than 2 mm is called sand. Within the classification of sand, you can also expect finer gradations. You can find a more detailed explanation of the topic here.

Something else that is important for the aquarist: the density of the substrate is related to the grain size. The finer the substrate, the higher the density. Larger particles therefore stand for a looser substrate. Fine sand is a very dense substrate.

Types of gravel

Basically, a distinction is made between natural and artificial types of gravel. While the former are mined in nature and come onto the market at most slightly pre-washed, artificial gravels are often treated in a more elaborate way during production. For example, gravel grains can be given a special color and then coated with synthetic resin. The resin protects against the color influencing the parameters in the aquarium water. This also makes very striking colors such as pink, purple, green, blue or red possible, which would not occur like this in nature. Gravels coated with synthetic resin are in any case water-neutral. Among the natural types of gravel there may sometimes be slightly water-hardening types of rock. Especially in combination with a CO₂ system, hardening can then take place. Without the input of carbon dioxide, this effect is almost negligible.

Furthermore, there are of course enormous differences in color, grain size and also the shape of the particles. The color of the gravel is primarily a visual matter; animals that shy away from light, for example, feel more comfortable on a dark substrate because it does not reflect light as strongly. Light-colored substrates, however, positively shine under a suitable aquarium lamp.
As for grain size, there is first of all the clear distinction from sand. As described above, a grain size below 2 mm is declared as sand according to the DIN regulation. Above that, the types are defined as gravel, with a further, finer subdivision into fine gravel, medium gravel and coarse gravel.
Which grain size you choose is, among other things, also a decision based on looks. There is also a biological role, however: coarser gravel creates larger gaps in the substrate. This promotes circulation in the substrate, but more detritus (mulm) can also accumulate in it, and the plants find it harder to get a hold in it. The pros and cons should therefore be weighed carefully against each other. You can also mix different grain sizes; aquascapers in particular use different types of substrate with different grain sizes to achieve great natural-looking effects that, for example, enhance the sense of depth and detail in the substrate area.

Basically, rounded particles are better for bottom dwellers so that there is no risk of injury from sharp-edged objects. This can be very decisive especially for bottom-sifting animals such as armored catfish. "Artificial" types of substrate in particular are therefore often rounded further during production in order to offer a shape that is very spherical. For this reason, some natural types of gravel in particular can be somewhat splintery to sharp-edged.

Preparing aquarium gravel

Depending on the product, the substrate should sometimes be prepared before it is added to the aquarium. This often means pre-washing the gravel. Natural types of substrate in particular have a higher proportion of dust and dirt that can otherwise cause cloudiness in the aquarium water. It is therefore worth washing the gravel out well under running water beforehand. Care is sometimes required when pre-washing products coated with synthetic resin. Here, warm or even hot water should preferably not be used so as not to affect the resin coating.

For pre-cleaning the gravel, all that is needed is clear fresh water and no cleaning agents at all. Usually the substrate is put outside into a suitable container that is allowed to overflow. The water is then added and the gravel stirred several times. The water now picks up the dust particles and becomes visibly dirty accordingly. You can now simply tip out the dirty water carefully or deliberately let the container overflow. Be sure to avoid substrate particles escaping from the container. You repeat this rinsing process until the water finally becomes as clear as possible. Afterward the gravel can be used in the aquarium.

Caring for gravel in the aquarium

Because gravel in a coarser grain size in particular offers quite a lot of gaps in the substrate, considerably more detritus (mulm) and dirt can accumulate here than with much denser sand. If the degree of contamination gets out of hand, it is advisable to remove this detritus from the substrate. Classic gravel vacuums come into play here, which can pull the dirt out of the substrate excellently without causing strong swirling and rearrangement near the bottom.

On the other hand, the detritus should not be removed too often either, since important pollutant-degrading bacteria live in it. An aquarium that is too clean often has a disturbed microbiology, and not infrequently an infestation with cyanobacteria (blue-green algae) occurs because there is a lack of microbiological competition. If too many beneficial bacteria are removed by cleaning actions, a rise in toxic substances such as ammonium and nitrite is even possible. For this reason, for example, cleaning work on the substrate and the filter should preferably not be carried out at the same time but with a clear time offset. This way, at least a certain portion of beneficial bacteria is retained.

Nutrients and aquarium gravel

Aquarium gravels are generally passive substrates that contain no nutrients at all. Using substrate fertilizers under the gravel layer is therefore recommended in order to be able to offer the plants important nutrients through the roots as well. Adding fertilizer balls or tabs is also a sensible measure, for example to support the growth of certain plants in a targeted way.

There are some exceptions, however. Many gravel and sand substrates from the company Seachem are rich in various nutrients. The so-called Flourite substrates differ greatly in grain size, color and nutrient composition and therefore offer a lot of options for finding a suitable gravel substrate for every aquarium.

How much gravel for my aquarium?

Are you planning a new setup and would like to know how much gravel or sand you need? No problem, it can be calculated at least approximately. It is important to know roughly the grain size of the substrate; then you can calculate the required amount using rules of thumb. More on this in this article.