The Hang-On Filter is a type of aquarium filter designed to be attached to the outside of a fish tank. It works by drawing water from the tank, filtering it through various media, and then returning the clean water back into the aquarium. This filter is popular among hobbyists for its ease of installation and maintenance. These filters typically use a combination of mechanical, chemical, and biological filtration methods to keep the water clean and healthy for aquatic life. They are suitable for both freshwater and saltwater tanks, making them versatile for different types of aquarium setups.