A diatom is a microscopic single-celled algae, which makes a bivalve shell by constructing cell walls composed of silicon. In fishkeeping the term is used in 'diatom filters', which use diatom shells as a filter medium through which water is passed. The diatom shells are extremely effective at removing fine particulate matter and reducing cloudy water. Diatom filters are rarely seen in modern fishkeeping, although they were relatively common in the past.