how are you dosing kalkwasser? because you should obviously try to dose the majority of it outside of the photoperiod, If using a controller like apex,, making a reverse photoperiod dosing schedule to keep pH elevated however throttled back enough not to cause alk swings
How are you adding kalkwasser? because if its just an unsealed container a ato or doser is pulling out of it can lose potency by reacting with atmospheric CO2, a sealed Nielsen reactor is a different story where water is pushed through via ato or doser / not exposed to ambiet CO2 and can be more potent as far as pH benefit is concerned.