A cleaning magnet is a two part magnet. One part goes on the inside of the aquarium, the other on the outside of the aquarium, and as magnets do, they mate together. The magnets are covered a felt type cover to prevent scratching. To clean the glass, you move the outside magnet and the inside magnet will follow. They are used to clean glass when you have limited access, such an in the wall aquarium, or when you don't want to get your hands wet. You can buy them at your lfs. My guess is $10 to $20.