WHAT? no, no, no! Carbonic acid lowers the pH, more c02 you add, the lower the pH goes. It is how fast the drop is that kills fish. Its called pH shock. It depends also on the fish and how sensitive they are to it. Ph swings over a period of several hours are gradual enough not to bother most fish.
If any one here has ever been into soft water fish, dwarf cichlids and the like and picked up fish at a club auction, you know you have to acclimate the fish to the water in your tank very very carefully. You very slowly add small amounts of your tank water into the bag of water with the fish over a period of an hour or more. This prevents the fish from going into pH or temperature shock. Its the same thing with C02. You raise the level of C02 slowly over a period of several hours. Co2 tank dump syndrome is when your C02 tank is near empty and it dumps the entire remaining contents into your aquarium. Your fish die from pH shock before they die from C02 poisoning
If your water has no buffers at all, virtually no KH, then your normal pH swings from morning to night will be much bigger and irradic
There is nothing like a good debate to keep the blood flowing!
The problem with your position, Robert, is that so many of us do 50% and greater water changes weekly, dumping in fresh tap water, with Prime added, but with whatever pH that water has. And, most tap water has a high pH. That very suddenly raises the tank pH quite a bit. But, the fish thrive on such changes. So, pH shock is very likely to be a myth.
Also, adding CO2 by any means available to us, does not rapidly drop the pH. We just can't get CO2 dissolved into the water that fast. The pH change occurs gradually, most likely over a 2 hour or so period. That is not a sudden pH shock.
Last, when we condition our fish, just purchased from the LFS, it isn't the pH change that we are protecting against, but the other water parameters changing rapidly. LFS may have much higher KH or GH than our tank has, or our tank may have much more tannins than the LFS has. It is very likely that those are the sudden changes we have to guard against.