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.

There are various configurations of external filter systems. An important feature is that it should be able to 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 maximizes contact time with the filter media. This is easily evaluated when observing how and where water flow is directed within the filter.

Fresh water aquariums generally require a minimum water turnover rate of approximately four times an hour. Marine aquariums usually do best with water turnover rates of 7 to 10 times per hour. Exceptions do exist with respect to the livestock being kept. In general, it is best to select a system (s) which provides somewhat more than the minimum rate. This will help compensate for declining flow rates as the filter system accumulates debris.