External filters are devices used in various systems, such as aquariums and air purifiers, to remove impurities from water or air. They work by drawing in the fluid, passing it through a filtering medium, and then releasing the cleaned fluid back into the environment. This process helps maintain a healthy ecosystem in aquariums or improves indoor air quality. These filters can be mechanical, chemical, or biological. Mechanical filters physically trap particles, while chemical filters remove contaminants through reactions. Biological filters utilize beneficial bacteria to break down harmful substances, making external filters essential for maintaining balance in systems like aquariums and HVAC systems.