These systems represent a common choice for most aquarium keepers. The general principle of operation involves the intake of aquarium water via a siphon that results in filtration through one or more filter media. The return flow is directed at the surface to provide oxygenation.

These are various configurations of external filter systems. An important mechanically, biologically and chemically filter water. In order to achieve maximum effect from the filter media installed, it is important to select a design that maximized contact time with the filter media. This is easily evaluated when observing how and where the water flow is directed within the filter. Fresh water aquariums require a minimum water turnover rate of approximately four times an hour. Exceptions do exist with respect to the livestock being kept. In general, it is best to select a system which provides somewhat more than the minimum rate. This will help compensate for declining flow rates as the filter system accumulates debris.