An external filter is a device used in aquariums to maintain water quality by removing impurities and debris. Unlike internal filters, which are placed inside the tank, external filters are located outside the aquarium and use a pump to draw water through a filtration system. This setup allows for larger filter media, improving the efficiency of water cleaning. External filters typically consist of multiple stages, including mechanical, chemical, and biological filtration. They can handle larger volumes of water and are often quieter than internal options. Many aquarists prefer external filters for their effectiveness and ease of maintenance, making them a popular choice for both freshwater and saltwater tanks.