RO water chemistry questions

I've decided to go completely RO water and remineralize with SaltyShrimp Shrimp Mineral GH/KH+. (I've written about my terrible tap water before.) I realize that RO water is unstable and the pH variable, in the low 6s as is.

Here's the thing. When I mix the water and SaltyShrimp to a TDS of ~190, KH of 5, GH of 9 the pH - even after sitting for a few hours - is about 7.2.

Overnight the (empty) aquarium's pH went up to 8.

So... every time I do a water change I'll be adding in water that is more acidic than normal. How do you mitigate this?