There is a huge problem right there, using kalk
to raise pH. You are throwing your parameters out of whack by using a product that will change your alkalinity. The pH change is just a side effect.
Either way, pH being affected by the winter months isn't a problem. This isn't a 'number' that is meant to be the same all the time, it even changes as the day goes on. If you are concerned about pH due to elevated CO2
in the house because no windows are open, look at a more natural way of CO2
exchange. Look at pointing a powerhead at the surface for more oxygen or even put a skimmer air hose out a window.
Might not be options, but better than dosing for pH as it always leads to larger issues. That said, my reef tank is years old at this point and is just as shut in due to my PA winters. I've never tried to address the pH and there are no issues.