Aquarium substrates are the materials placed at the bottom of an aquarium, serving multiple purposes. They provide a surface for beneficial bacteria to grow, which helps in breaking down waste and maintaining water quality. Common types of substrates include gravel, sand, and soil, each offering different benefits depending on the type of aquarium and the species of fish or plants being kept. Choosing the right substrate is essential for the health of the aquarium's ecosystem. For example, planted tanks often benefit from nutrient-rich substrates, while cichlid tanks may require larger gravel to prevent digging. The substrate also affects the aesthetic appeal of the aquarium, contributing to its overall design.