Features: NEW! Controller programming circuitry for added system protection NEW! Improved sensor design for longer durability World's smallest dual-sensor ATO Two layers overfill protection Intelligent QST programming Secondary optical sensor Super precise water level control Super easy installation; plug-and-play No programming or adjustments required Works in complete darkness With huge features jammed in a tiny package, the XP Aqua Duetto 2 Dual-Sensor Aquarium Auto Top Off System is the perfect ATO for your aquarium. XPAqua?s proprietary dual-optical sensor packs two precise level sensors in one incredibly small package. In addition to keeping your water level constant, the two sensors also work in concert with the intelligent QST programming to help prevent overfilling. All the installation parts are included and a magnetic coupling allows you to easily mount or adjust the dual-sensor. Installation takes just seconds so you can spend less time working on your aquarium and spend more time enjoying your aquarium.