[mr_alberta]

Basically you have a switch has a "bobber" that floats up and down in your tank. When the water level in your tank drops below a certain level, the switch "closes" and turns on your powerhead. The powerhead, which is located in the rubbermaid full of fresh water then pumps water back into the tank. As the water level in your tank rises, the "bobber" on the float switch rises and "opens" and cuts off the power going to the powerhead.