This is perfect. Thanks OP for opening this topic. I was logging on today to find a solution exactly like this. In my case I am installing my second res on a shelf above the first res. I will use some sort of float valve to control a gravity fed line to the res below. To get water to the top res I will use a mag drive pump capable of head pressure of at least 6 feet to pump the water upward. I think I will also install a timer to cycle the water every hour or so. I think a float valve is exactly what I need to make this work. I was also thinking along the lines of using a toilet float valve but hadn't yet figure how to make that work. Some how the bottom tank needs to have this float valve attached and I'm not sure how to make that happen. It didn't occur to me to think along the lines of livestock water tank valves.