Diatom filters are used to "polish" water or spot clean an aquarium. The filter utilises diatomaceous earth to form a fine filter on a screen. Water is forced against the screen and particulate waste is trapped. As the filter clogs, the finer the screen becomes and thus it traps smaller particles. The screen can quickly become so clogged that it will trap particles as minute as one micron. Since the filter traps water so efficiently the flow rate through the screen drops dramatically and the screen must be replaced. It is for this reason that diatom filters are only used short term and are not a permanent filter for the aquarium.

Due to the short periods that diatom filters are used, there is no opportunity for bacterial to develop and therefore they are probably the only purely mechanical filter available.

One should consider before purchasing a diatom filter what is causing the water to require such rigorous cleaning. Generally, a well sustained aquarium with conventional filtration will maintain crystal clear water and cloudy water is the cause of an underlying problem.