If the swing is due to the CO2 injection, this has no effect at all on the fish.
Their blood is loaded witgh CO2, just like ours and the gills exchange the CO2 for O2.
pH shifts due to salts, eg, adding a lot of baking soda or removing it rapidly etc, these can cause pH shifts, but the death/stress is not due to the pH, it's do to osmotic shock.
Sort of like tossing a fish into salt water or from salt water to freshwater.
Add some dechlorinator as you are filling the tank.
Otherwise make sure the temp is the same upon refilling and if you have not done many water changes prior, sometimes fish are stressed, but more water changes tend to make the tank very stable and then you can do large, 80-90% water changes without issue if you want.
I have altums, rare SA soft water fish etc, I do huge weekly water changes for them and have done 50% or close for about 30 years and I have a long list of fish I've had without ever a disease, never needing medications, having bred about 20or so species etc.
I have clients with perhaps the entire net worth of the LFS's livestock in their tanks, I do 50-70% water changes weekly without issues.
So have most everyone that does EI and have for many years now.
So it's something else you are overlooking, not the water change or pH.
Get a tap water report also, that can help in many cases and is a wise idea.