Substrate is the term used to refer to whatever material is used to cover the bottom of your fish tank. The substrate plays a part in the filtration, water chemistry and the general health and wellness of your tank's inhabitants. Additionally, substrate plays an important role in the aesthetic appeal of the aquarium. Decisions about substrate are based upon the fish type, size, aquarium type and whether the fish tank contains saltwater or fresh water.

Fresh water tanks typically use fish gravel as the substrate of choice. This gravel, however, must meet a number of requirements in order to properly function as the tank's substrate. First, the gravel must not have sharp edges. The fish gravel can range in size from as large as peas or as fine as one mm. The optimal size is often dependent on whether your fish swim or sit at the bottom of your tank. The gravel comes in many materials, but it is typically comprised of quartz and other lime-free materials. Additionally, some gravels have different Cation Exchange Capacity (CEC) levels, which affects the ability of the gravel to sustain plant life. Many types of gravel are available, and you can find out more about them here.