Potential Low pH problem and how to fix: If after running the Kalk / Calc Combo Method for a few days, your tank pH is below 7.9 during the day, I would recommend doing a couple of things. First, I would add a second reaction chamber to the calcium reactor if it does not already have one. The second chamber contains reactor media, is very simple and only has one in and one out. You run the effluent out of your calcium reactor and into the secondary reactor. This reactor removes CO2 from the effluent by dissolving additional media. This increases the pH of the effluent entering the tank. Another thing you can try is to drip the effluent into a container that has an air stone in it and then overflows into the sump. This again degasses some of the CO2 and further increases effluent pH before entering the tank. Here is a schematic of a system including the second chamber and degassing container.
The schematic below shows an alternate calcium reactor setup that helps raise pH.