I would not recommend using kalkwasser with tap water. Not necessarily because the combination of the 2 will cause any problems, but because your problem can probably be solved using other, better means.
Kalkwasser is generally used to raise pH in systems running a calcium reactor. Calcium reactors increase CO2 levels in an aquarium. The higher the CO2 level the lower the pH in an aquarium. CO2 and pH are inversely related.
If you have no calcium reactor and a low pH and that is why you are considering using kalk, there are probably better remedies. First of all, stop using tap water! If your water is acidic, every time you do a water change your pH will be lowered. Use RO/DI water or distilled water. Second, increase water flow. The more flow in your tank the more gas exchange. This will reduce CO2 levels and increase oxygen levels, thus raising pH. Finally, if you still have low pH, check your alkalinity, Alkalinity and pH are directly related. If your alk is low you can raise it using baking soda, thus raising your ph.
If you want to use kalk to raise your calcium level there is a better option. Use calcium chloride or a 2 part solution.