I had the same quetion and so instead of doing something really complicated I did something simple. It cost me a bit more money but Its worth it. I got a top off system that has a tube that hangs in the tank, it detects the waterlevel. THe tube is hooked up to a box that you can plug a ph into. When the level is low the Power Head turns on and takes the water from your resevoir tank into your main. I took an L shaped tube and have it hang over my tank, and to solve the problem of the tank water syphoning into my reservoir tank the tube is 5inches above the water level when its at its highest. My total "rig" cost me about 80$.