No, it is not recommended to leave the magnet cleaner in the tank. Magnet cleaners are designed to be removed from water after each use in order to avoid any buildup of debris on the surface of your aquarium, which can lead to problems such as decreased oxygen levels and an increase in algae growth. Additionally, if left inside for long periods of time they may corrode or rust and contaminate your water with metals that could prove toxic for fish and other aquatic life. Instead, you should remove them when cleaning your tank and store them away until their next use.