There are several potential reasons associated with your low pH, but it likely has to do with having too much carbon dioxide in the tank water. My suggestions:
1) Route the air intake to your skimmer, assuming you are using one, to draw in air from the outside.
2) modify the configuration of powerheads in your aquarium to increase surface agitation at the surface of the aquarium (to help with air exchange)
3) make sure you are dosing kalk slowly over time, and the the location you are dosing it has plenty of flow. If not, you could just be precipitating calcium carbonate as quickly as you are adding the kalk.
4) run your air intake to the skimmer through a carbon dioxide adsorption media. This will work, but may not be cost effective in the end.
Please note the kalk can, and usually does, increase pH in the aquarium but that increase is typically not more than 0.1 to 0.2 pH units.
I share your pain here. I used to live in a home that was very air tight, and had chronic pH problems due to having too much carbon dioxide in the air. In my opinion the best strategy, and the one I used, was to oversize my protein skimmer and run the intake though the wall to the outside.
