Here's an "acid" test... pardon the pun.
Turn off the CO2 and leave it off for 24 hours.
Then after it's been off for a day, buy some fish.
Put them in.
Continue to leave the CO2 off for another day or two or three (or until the fish die if they do).
If the fish die, you KNOW it isn't CO2 caused pH swings that's killing them.
If the fish don't die you've got a big red flashing sign that says "pH Swing is the problem."
You can turn the CO2 back on at that point and let it swing like you have been doing. Exactly the same, no changes or you spoil the experiment.
One of two things will happen... The fish will then die (which will turn on the siren over the red flashing sign)
-or- they will have gotten over the initial shock of being transplanted and may be healthy enough to stand the swings and live.
That would tell you once and for all...