Ther JBJ ATO has two sensors and can be configured in several ways. I have one sensor in the main tank and one in the fresh water reservoir. If the reservoir runs dry the pump will not activate so the pump doesn't run while dry. I paid $80 with shipping.
i use a ten gallon aquarium next to my sump with jbj ato, ten gallons last a week in my 125 gallon. So i only refill once a week. gonna add a bigger holding tank so i can go longer.