Aquarium substrates or gravel fulfills a few functions for us as aquarists. Gravel/sand provides living space for beneficial microbes, perhaps alkaline reserve and other chemicals for a system. There may be behavioral benefits for your livestock, anchoring for plants... and, it looks good too. The majority of aquarists use some form of substrate in their tanks, though strictly speaking they could leave the bottom bare in most cases. Here is our discussion of the Uses, Properties, Varieties, and Application/Maintenance of substrates for freshwater aquariums.

Uses: Biological, Buffering, Biomineral, Psychological, Looks

1) Bacteria Homes: 

First and foremost, gravels, sands and other solid décor in aquariums function to support populations of beneficial bacteria. There is a definitive relationship between the microbes and macrobes (livestock) in your aquarium. The most celebrated of these 'bio-geo-chemical cycles' is the element Nitrogen.

Well, where do all these bacteria 'live'? Mainly in and around the substrate, though they are found on the tank walls, decor, even on the inhabitants... You do need them… to provide means to limit or remove the poisoning waste products and by-products of your livestock.

Many/most aquarists opt for the 'gravel route' in their tanks or filters as the least expensive, most secure method of getting rid of metabolites and providing a ...

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