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Brando is correct. I struggle with similar issues. There is often magnesium carbonate, and or calcium carbonate in water. These are the main cause of ‘hardness’ and ‘total alkalinity’. When these are present the ph will drift up over time. I used some test strips to better understand what is in my well water. You can get a water report from your utility company. Those aren’t as detailed.

RO and distillation are the most effective ways to remove calcium carbonate. Most other filter types won’t. For this reason people blend RO with other water source 50:50 to reduce the concentration. Theoretically if you had the right concentration and proportion it wouldn’t drift or lock as much.