I have the Tunze auto-top off and use a Kalk reacot in combination with it. The Tunze pump sits in my resevoir (A 44 quart dog-food storage container from Petsmart.) When the pump kicks on it pushes water into my PM 660 Kalk reactor (which I really love and holds a TON of kalk) and then up and into the overflow box in my tank. I put a check-vave in the line in between the pump and the Kalk-reactor to prevent a back-siphon and everything has been working fine for a month now.