When it comes to auto top off units, XP Aqua, Neptune Systems, and Auto Aqua are 3 names that mean dependability. ATO units are one of the smallest and most technologically advanced pieces of equipment that your tank needs. The way that ATOs work is an "eye" or sensor, usually about the size of a quarter, reads the water level in your sump. When the water level gets too low the sensor then tells the pump to switch on and draw water from a separate reservoir to replenish the water that has evaporated from your tank. Please remember to always use fresh, filtered water for your ATO reservoir and never saltwater. If you have any questions about which unit is right for you don't hesitate to contact us.