The material used as the "seabed" in an aquarium is referred to as substrate. Gravel is the most common type of substrate.

Gravel is a popular choice to cover the bottom of aquariums, and give the fish something to nose about in for food. The gravel must be clean of foreign matter and impurities that might harm your fish, and can be bought from your nearby pet shop or aquarium retailer. Gravel is available in different shapes, sizes and colours, and is fairly easy to clean each week. Sand is not as popular as gravel, but has a more natural look and is easier to clean because waste tends to gather on the surface. Care must be taken to avoid the sand being sucked into the filter’s intake.

Coloured glass beads can add to your aquarium’s appeal. They are perfectly safe and have replaced dyed gravel, which was popular in pet shops and home aquariums but which slowly leeched the dye into the water. If you have live plants in your aquarium, consider buying appropriate fertiliser and laying it under the gravel. The main types are fluorite and laterite (both clay-type substances) or vermiculate (a mineral).

Sea sand, crushed coral and aragonite (calcium carbonate) can be used to enhance saltwater aquariums but are not suitable for freshwater tanks because they may contain micro organisms which will pollute the water, and raise the pH level. It is important to clean your substrate each week, as food particles and fish waste can easily accumulate and add toxins such as ammonia to the water.