We use CO2, which fish have in their blood and regulate rapidly, vs salts, like Alkalinity, HCO3, which they do regulate, but it does not appear and diiffuse out in a similar fashion.

Put another more practical way:

Take a large tank using CO2, the KH is say 5, the pH is say 6.6 and a GH of say 9.
Do a large water change, say 50-70%.
The incoming non CO2 enriched tap has a pH of 7.9.
KH for the tank and the tap is still KH= 5.

Result: fish are always fine.

Conclusion, pH shock is a myth.
It's not pH. It's the difference in salts in the KH mostly and the GH perhaps.

Next, try a KH of 1 and a GH of 1: results: generally not too bad. Fish can slowly acclimate from hard to soft.

Next, try from soft to hard, go from KH= 1 toa KH of 10.

Do this rapidly: dead fish.

pH can be kept the extact same level using CO2 or other acids or buffers for both treatments.

The ability of the fish to adapt to salts, rather than pH is the real issue here.

When fish only folks discuss pH, they usually mean KH differences, as they correlate with pH, more KH= more pH.

But when we add CO2, or look farther into the real issue, we find it's not pH by and of itself.

This can cause confusion and start heated debates as one person claims what they have been taught for 30 years must be right, while another that test and sets up practical test can clearly demonstrate it cannot be due to pH alone.

Some folks do not like me for this approach, nor questioning basic questions in the hobby, but I'd prefer to be right and have a better understanding than a big ego/and yell louder than the next person and take it personally as many often do.
Watch, see who actually answers the questions about the topic and who avoids them.

Do not be swayed by belief or weasel words.
Stick to what makes the most sense.
That's how you learn and that is always on topic.

Regards,
Tom Barr