Thanks everybody for all your input, I do appreciate it.
Just to be clear, what I'm looking for isn't something to automatically top off my tank. All I'm looking for is a mechanism to prevent my pump from running dry if for some reason one of my U-tubes loses its siphon and the refugium where my pump is at loses water.
In that picture, it's the tank underneath on the right where I want to install this.
Maybe one of the top-off systems would work when mounted upside-down, but the goal here is not to regulate the water level in the display tank, I have a HOB
overflow cup system to do that and I'm happy with it.
The one thing that's missing from this picture for me is a way to hook up the rest of the circuit that dezso posted to the 120V power source safely. Right now the only thing I can do is to stick the wires into the wall socket, which I really don't want to do. Is there such a part that I can plug into the wall and attach wires to?