Fill water and chemistry

[anonapersona]

I just measured my fill water, straight from the tap and after sitting for 24 hours. I know that straight from the tap will have a lower pH than later after the dissolved CO2 is released as the water goes from water main pressures to atmospheric pressure.
FC 0
TA 220
CH 20
pH 7.9 from tap
pH 8.4 maybe after 24 hours, hard to be sure color was above my testing kit max at 8.2, guessing at 8.4

I notice that when I put these values into the Pool Calculator, using pH of 8.4, that the CSI = 0 exactly.

Is this some sort of universal truth or just a coincidence? At rest, water will balance out the pH to make CSI= 0 at a given TA and CH? Just wondering..... and wondering what that says for chemistry post resurfacing the pool.

Chemistry is not my long suit and so while I think about it and try to understand, I need to have simple explanations and pictures to make it make sense to me.

 

[JasonLion]

The CSI coming out at zero is just coincidence. CSI starts negative, and with TA that high the PH will continue to go up until CSI is positive. Most places the tap water has strongly negative CSI. Your high TA tap water just happens to bring you into the range where you will cross over into positive CSI if you let it sit out.