External filter (canister)
The most used type of filter in freshwater aquariums. A pressurized container outside the aquarium that houses biological, mechanical and chemical filter materials.
Detailed explanation
The external filter (canister) is the heart of filtration in freshwater aquariums. A watertight container positioned in the cabinet under the aquarium sucks in the water, passes it through the filter materials and returns it to the aquarium.
The filter materials are arranged in layers: coarse sponge, ceramic rings (biological filtration), fine sponge, possibly activated carbon.
The most reliable brands are: Eheim (Classic and Professional), Oase, JBL CristalProfi, Sera fil Bioactive.
The sizing foresees a flow rate of 3-5 times the volume of the aquarium per hour. Maintenance must be done every 4-8 weeks by rinsing the sponges in aquarium water.
Never turn off the filter for more than 2 hours: the bacteria inside consume oxygen and in anaerobic conditions produce toxic hydrogen sulphide.