Substrate (aquarium) - Types of Substrate

Types of Substrate

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-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 adsorb 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.

Shell grit, crushed limestone, crushed marble, crushed coral skeletons, coral sand, and oolitic aragonite 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 cichlids or for saltwater fish and invertebrates. Some invertebrates such as mollusks or stony corals also use the calcium 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 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 catfish, peat is reported to have a number of other beneficial functions in freshwater aquaria. It softens water by acting as an ion exchanger, 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 tannins.

Sand is often recommended for use with certain species, such as the river stingrays of family Potamotrygonidae, which bury themselves in the fine substrate. However, these species can be successfully kept with coarser substrates as well.

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.