An aquarium filter is a device used to maintain clean and healthy water in fish tanks. It works by removing debris, waste, and harmful chemicals from the water, ensuring a safe environment for aquatic life. Filters can be mechanical, biological, or chemical, each serving a specific purpose in the filtration process. There are various types of aquarium filters, including hang-on-back, canister, and sponge filters. The choice of filter depends on the size of the aquarium and the needs of the fish and plants inside. Regular maintenance of the filter is essential for optimal performance and water quality.