Thanks so much for the welcome and your message.
I tested the CYA again twice:
After mixing 1 part pool water and 1 part tap, CYA 75
After mixing 1 part pool water, 2 parts tap water, CYA was between 40-50
So should I empty 2/3 of the pool?
I measured the CH, it is now between 450 and 475
When...