External filters for aquaria need to have sufficient reserve performance capacities in order to provide for a suitable aquarium water circulation at all times. A sufficiently strong current disperses the nutrients equally in the entire tank. Plus, CO2 is also transported even to the inside of dense aquatic plant populations.
Moreover, external filters remove noxious substances from the water, thus improving water quality significantly and providing the tank inhabitants with clear, clean water.
Another advantage of an external filter is its location outside the tank. It leaves aquascapers more freedom when decorating their tanks, as they are not visible inside the aquarium, as an internal filter would be. Internal filters take up a lot of space in an aquarium, and they are difficult to hide behind aquatic plants or hardscape.
For aquascapers, an external filter is always the preferred choice.