Add Calcium to your DI water

[RuGlu6]

My RO/DI water is "0" TDS
Tap water is 13 TDS some times 15 TDS when its raining.
Tap water Ph 8.5 but as soon as in the glass and as soon as i start swirling the Ph probe the Ph goes down to 7.0 in 30 seconds.

RO/DI water Ph is 9.0(!) but with the same almost instant Ph drop down to 6.9 after ph probe swirling in the glass.

Did anyone else tested their RO/DI water Ph?
Or even better Ph INSIDE the water filter?