The substrate of an aquarium refers to the material used on the tank bottom. It can affect water chemistry, filtration, and the well-being of the aquarium's inhabitants, and is also an important part of the aquarium's aesthetic appeal. The appropriate substrate depends on the type of aquarium; the most important parameter is whether the aquarium contains fresh water or

saltwater Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish water, ...

Functions and considerations

Substrates are added to most aquaria principally for the increase in beneficial bacteria this provides. However, substrates can also have a variety of direct effects on water quality by releasing substances into the water, absorbing substances from the water, or reacting chemically with substances from other sources. Substrates can also have indirect effects on a system's health; dark-colored substrates, for example, are considered by some to be better for fish, as the fish display more colorfully by comparison, and are less likely to behave timidly. Apart from all other considerations, substrates are frequently chosen for their aesthetic qualities. Some substrates are used to alter water chemistry. Crushed coral and coral sand both contain calcium carbonate, which will raise the carbonate hardness and buffer the pH.

Peat Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficient ...

may be used in some aquaria to mimic some soft water habitats. Substrate may also be used as part of a biological filtration system. Beneficial bacteria colonize all aquarium surfaces that are exposed to aerated water, including the substrate. Because the numerous particles have a high surface area, substrates are often employed in biological filtration. Some common types of filtration involving the substrate include the undergravel filter and the deep sand bed. Planted tanks require a substrate that will remain loose enough for plant roots to penetrate it. The substrate should be chemically inert and free of sharp edges. Fine gravel (1–2 mm) is preferred by some aquarists because coarser substrates allow debris to settle within the gaps between grains, which is particularly difficult to clean in a planted aquarium. Sloping the substrate so it is most shallow in front accommodates larger plants with correspondingly larger root systems in the back. The substrate for plants should be at least 5 cm (2 in) deep. Often, a lower layer of richer substrate such as potting soil, peat, vermiculite, or certain types of clay are used as a source of iron and trace elements for plant roots. In breeding tanks for egg-scattering species, a layer of marbles is sometimes used as a substrate, allowing the eggs to fall into the gaps between the marbles where the parents cannot eat them. Quarantine tanks (sometimes called hospital tanks) often use no substrate at all. This assists in keeping the aquarium as clean as possible as well as monitoring of fish excrements. Fish of the species ''Pelvicachromis taeniatus'' were shown to be able to recognize olfactory traces left behind by substrate, even in clear water; highlighting that the presence of substrate is important for fish kept in tanks. This method is beneficial to increase fish activity drastically in empty tanks.

Types of substrate

Gravel

For freshwater aquaria, gravel is the most common substrate. To prevent damage to fish, gravel should not be sharp. Aquarium gravel can be as coarse as pea-sized or as fine as 1–2 mm. It is available in a number of colors, and may be naturally colored or dyed, and may have a polymer seal to ensure it does not affect water chemistry. Gravel sold specifically for use in aquaria is chemically inert. It is commonly composed of quartz or other

lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...

-free minerals. If the gravel is rough or sharp, it is not suitable for bottom-dwelling fish that like to sift the substrate or dig. When growing aquatic plants, the Cation Exchange Capacity (CEC) is also an important thing to consider when choosing a substrate. CEC is the ability to absorb positively charged nutrient ions (so high CEC is good). This means the substrate will hold nutrients and make them available for the plant roots. It doesn't indicate the amount of nutrients the substrate contains.

Minerals, Shells and Corals

Shell grit, crushed

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

, crushed

marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Marble is typically not foliated (layered), although there are exceptions. In geology, the term ''marble'' refers to metamorphose ...

, crushed

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...

skeletons, coral sand, and oolitic

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...

substrates are possible choices. Because calcium carbonate, the primary component of these substrates, increases water hardness and pH, these are used most often for hard water species, such as those for African

rift lake A rift lake is a lake formed as a result of subsidence related to movement on faults within a rift zone, an area of extensional tectonics in the continental crust. They are often found within rift valleys and may be very deep. Rift lakes may be ...

cichlids Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted th ...

or for

fish and invertebrates. Some invertebrates such as

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s or

stony coral Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mo ...

s also use the

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

and strontium released. Calcium carbonate substrates are poorly suited to aquaria housing most other freshwater aquarium fish, particularly river species, which are adapted to soft water.

Peat

, or decomposed plant matter, is used most commonly in soft water or

blackwater river A blackwater river is a type of river with a slow-moving channel flowing through forested swamps or wetlands. As vegetation decays, tannins leach into the water, making a transparent, acidic water that is darkly stained, resembling black tea ...

systems, such as those mimicking the Amazon River basin. In addition to being soft in texture and therefore suitable for demersal (bottom-dwelling) species such as ''

Corydoras ''Corydoras'' is a genus of freshwater catfish in the family Callichthyidae and subfamily Corydoradinae. The species usually have more restricted areas of endemism than other callichthyids, but the area of distribution of the entire genus almost ...

'' catfish, peat is reported to have a number of other beneficial functions in freshwater aquaria. It softens water by acting as an

ion exchange Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...

r, it contains substances good for plants and for the reproductive health of fishes, and can even prevent algae growth and kill microorganisms. Peat often stains the water yellow or brown due to the leaching of

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...

Sand

Sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...

is often recommended for use with certain species, such as the river stingrays of family

Potamotrygonidae River stingrays or freshwater stingrays are Neotropical freshwater fishes of the family Potamotrygonidae in the order Myliobatiformes, one of the four orders of batoids, cartilaginous fishes related to sharks. They are found in rivers in tropic ...

, which bury themselves in the fine substrate. However, these species can be successfully kept with coarser substrates as well.

Clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

can be mixed in with sand to provide extra nutrients for live plants.

Leaf (leaves)

A growing aquarist substrate is the use of leaves in an aquarium in replacement for traditional substate. The leaf or leaves can be mixed with other substrates or solely on their own. They mimic the natural environments of aquatic creatures and create a stress free and pH balancing environment for flora and fauna by tinting the water column colour to a reddish hue with the release of beneficial tannin that certain aquatic life prefer. They are often used in blackwater aquarium biotopes. Care has to be taken as some leaf matter may be poisonous in nature and harmful to aquatic life. In some aquaria, different substrates are used in different parts of the tank. For example, peat can be used in one corner, while gravel in another portion allows rooted plants.

References

{{Aquarium Fishkeeping