Substrate, or the bottom layer of your aquarium, serves an important purpose outside of the beauty it adds to the aquarium. It is in the substrate that much of the aquarium's beneficial bacteria will live, so it actually helps keep your fish healthy. Choosing the substrate and putting it into the aquarium correctly is the start of creating a healthy environment for your new fish.
Type of Substrate
If your aquarium is a "fish only" aquarium, gravel works well as a substrate. It comes in a variety of sizes, and size only matters if you have bottom feeders in your tank. If you do, choose fine gravel so they will not choke. If you have a planted aquarium, choose a substrate, like sand or laterite, that can store plant nutrients. The sand used in an freshwater aquarium should be play sand or sandblasting sand, not sea sand. You can layer substrate if you wish to have decorative gravel on the top and a cheaper alternative on the bottom.
How Much Substrate?
The amount of substrate is also important. Too little substrate will not provide enough room for beneficial bacteria to grow, but too much substrate can allow food particles to become trapped in deep layers where there is less oxygen. This can lead to the production of hydrogen sulfite, which will kill your fish. You want your substrate to be between two and two-and-a-half inches in height. This is typically 10 pounds per 150 square inches of surface area in your tank.
Putting the Substrate in Place
Once you have chosen your substrate, carefully clean it before adding it to the fish tank, unless you are using sand. All granular substrate, like gravel, needs to be cleaned. Look through it for any sharp pieces or large foreign objects, and remove these. Carefully place it in an empty tank.
If you are planting your tank, slope your substrate slightly so that it is higher in the back of the tank and lower in the front. This will give you a place to plant larger plants. Otherwise, a flat layer is fine. Add our water slowly so as to not displace the substrate, and give the tank a day or two for the substrate to settle before adding your fish.