I use a kalkwasser [lime water] reactor. I put powdered lime into a cylinder. My top off water flows through, a laboratory stirrer keeps the lime water slightly stirred, and ro/di water absorbs just enough lime to bring the right amount to the tank. As tank water evaporates, the float switch causes the top off water to flow through the kalk reactor, which is only a mixing area. The water flows out of it into my tank, and it maintains alkalinity and calcium continually. You must establish the right level of alkalinity and calcium by hand-adding buffer and calcium before using the kalk reactor, but once it is set, you are ok to continue with the kalk reactor for as much as a month without having to dose any more. You test to be sure, but my tank [56g] is doing fine with only that, for the last 2 months.