Diatoms are classed as eukaryotic algae and are common type of phytoplankton. Most diatoms have a unicellular structure although they can have various shapes when they collect in colonies, for example, ribbons, stellates, fans, zigzags, or filaments. Diatoms have a characteristic encasement within a cell wall called a frustule made of silica, which is the chemical compound hydrate silicon dioxide SiO2 and is commonly found in nature as sand and quartz along with diatoms.
Diatoms are producers, meaning they are a primary food source within the food chain (similar to plants being a primary food source), with fossil evidence of their existence dating as far back as the Jurassic Period. Fast-growing species in productive environments build up biomass until grazing pressure leads to niche iron depletion, whereas large diatoms are found in areas where grazing pressure is heavy relative to niche iron limits. Diatoms are most often sold commercially for aquarium use. PetStore.com sells 1 pound bags of diatoms as hydrated amorphous silica (or opal), which it recommends for aquariums.