Arguably the most popular of filters, the external power filter is versatile enough to provide mechanical, biological and chemical filter functions. Some more advanced external filters even incorporate a heating element which negates the need for in-tank heating.

External power filters essentially consist of a series of baskets housed inside a cylinder. Each basket can be filled with a variety of filter media to suit the needs of the aquarium. Water is pumped electrically from the tank and flowed through each basket before being actively pumped back to the tank. The return flow can be restricted through the use of a spray-bar, for calmer aquariums, or left "open" to create a current. As varying grades of media can be housed in each basket, the finer grades will require cleaning less frequently. Cleaning can also be minimised to partly removing media from one basket at a time thus one can constantly maintain a vast biological population.

The external power filters capacity, up to 10 times that of an internal filter, means that it is ideal for large, densely stocked or messy aquariums although smaller models are available for more modest tanks. Another great advantage over internal filters is that external filters do not take up valuable tank space - they can be sited out of sight underneath or beside the tank.