I hate my ATO

Well those are not really ment to be gravity fed. You could though buy the swtich(one linked to) and an electronic solenoid which can be found on the site. You would install the solenoid in line with your feed line from the 8 gallon reservoir and plug it into the receptacles with the float switch. When the water level got low the switch would turn power on which would turn on the solenoid and opening up the line for water to flow. This may be overly complicated then you want to do though.