Sand, just like gravel and soil, is one of the most popular substrates in the aquarium hobby. What would a South American aquarium be without a sand bottom? In aquascaping, too, sand is often used as a decorative element. In this article we explain what advantages sand has and what to keep in mind when handling sand in the aquarium.

Grain size

Basically, sand is a great substrate for an aquarium that is very fine and light. Bottom-sifting animals such as armored catfish in particular love this substrate. Moreover, because of the shape and small size of the particles, it is harmless and has no sharp edges on which animals could sometimes injure themselves.
By definition, however, the transition between sand and gravel is fluid, so it depends very much on the so-called grain size. If the particle size is smaller than 2 mm, it is called sand, and above that it is called gravel. There are further gradations within the classification of sand as well. A grain size of 0.63–2 mm is called coarse sand, and a grain size of 0.2-0.63 mm is medium sand. Fine sand is defined by a particle size of 0.063-0.2 mm.

Because of its small grain size, sand has a higher density than gravel. Consequently, a sand bottom in the aquarium has significantly fewer gaps and voids. This slightly reduces the flow through the substrate, but it also has the advantage that these gaps do not clog up as quickly with detritus (mulm) and other organic waste. Often the "dirt" in the aquarium simply stays lying on the sand surface and can easily be vacuumed up with a hose during maintenance. If organic waste still accumulates in the lower substrate layers, however, there is sometimes unfortunately a risk of putrefaction gases forming with sand. The aquarist should therefore check this regularly (poke around in the substrate with a long stick and watch whether air bubbles rise) and avoid as well as possible letting organic material get deeper into the substrate. For these reasons, using substrate fertilizers that contain organic components under sand substrates is also not really recommended. To support plant growth in a targeted way, better suited fertilizer tablets can be used.

Types of sand

Besides different grain sizes, and thus coarse sand, medium sand and fine sand, stores offer substrates above all in various colors. You can also distinguish between natural sand substrates and those that are produced artificially. Natural substrates are simply mined in nature and otherwise generally not processed further (sometimes pre-washed, however). Artificial sand substrates are mostly particles that have been dyed. To protect the color, the substrates are additionally coated with synthetic resin. This also makes very loud colors such as green, purple, red or blue available. Natural substrates tend to offer ordinary colors such as various shades of white, black, gray or brown.

Light-colored substrates in particular reflect light quite strongly and make aquatic plants and fish positively shine. For shy and rather skittish animals, however, a darker substrate is sometimes the better choice, especially if they are used to a dark bottom from their natural habitat.

For an aquascaper, sand lends itself above all to decorative zones, such as in the foreground or for use in small lanes or gorges. This is then referred to as a sand path. The sand is then used where the nutrient-rich soil substrate is not needed for the plants. This way, great aquarium designs can be created with different types of substrate. Keeping the two substrates well separated from each other takes a few tricks. You can learn more about it here.

Occasionally, a few natural substrates can harden the water slightly, for example when there are lime inclusions in the little stones. This effect is much stronger when a CO₂ system is used in the aquarium. Then KH, GH and the pH value may rise. Without the introduction of carbon dioxide, this effect is almost negligible. Otherwise, sand substrates are to be understood as passive substrates that otherwise have no influence on the water parameters in the aquarium.

Preparing aquarium sand

Similar to gravel, it can also be a good idea to pre-wash sand before adding it. This makes sense if the substrate still contains a number of dust particles, for example through abrasion, which could otherwise cloud the water. For this you need a suitable container that is large enough to hold the substrate, and preferably a place to set it up where the container can be deliberately allowed to overflow. For example, you can simply set up a bucket outside. You fill the container with the substrate and then let water in very carefully. Dust and other dirt particles are now washed to the water surface. You can now stir the substrate slowly and gently to intensify the cleaning process. Then you tip off the dirty surface water a little or, with the utmost care, deliberately let the container overflow in order to remove the dust. You repeat this process several times until the water in the container is as clear and clean as possible.

In contrast to the heavier gravel substrate, however, the utmost care is required, especially with very fine sand substrates. The small particles are extremely light and are in danger of simply being washed away during the cleaning processes. The aquarist should therefore always proceed very slowly and gently with the water movements.

Caring for sand in the aquarium

A flawless look of the sand takes some care. Over time, detritus, fish waste or plant remains settle on the surface of the sand layer. You should remove these regularly. This is easiest if they are vacuumed up with a hose during the weekly water change. A sufficient stock of shrimp and snails also ensures that as much dead organic material as possible is processed and broken down. If the sand is ever out of shape, you can smooth it again or reshape it very easily with a sand flattener, for example to restore slopes.

Nutrients in sand?

Sands and gravels are generally passive substrates that offer no nutrients at all for the aquarium plants. This is why a substrate fertilizer is often used under gravel. With fertilizer balls or tablets, sand can also be enriched with nutrients at specific spots, so that especially heavy root feeders such as Cryptocorynes can benefit greatly.
There are also some exceptions, however; the manufacturer Seachem, for example, also offers nutrient-containing sand substrates. The Flourite substrates differ in grain size, color and nutrient composition.

How much sand for my aquarium?

When setting up and planning an aquarium design, it is important to know how much substrate you need for your aquarium size. Unfortunately, this is not entirely easy, because the volume of the aquarium is given in liters, while substrates such as sand or gravel are generally given in kilograms in stores. If you know roughly the grain size of your substrate, however, you can calculate your sand requirement with the help of the following rule of thumb.