PH can change by two different pathways, via changes to levels of carbonic acid (caused by co2 injection) or via changes to levels of carbonate (caused by changing kh). Changes to the former pathway don't affect fish, but changes to the latter do. So we should avoid rapid swings in ph if they're caused by rapid changes in kh, but not worry about them if they're caused by changing levels of co2.
This actually makes sense because I always thought that pH swings were stressful toward fish too. My tank is also CO2 and it was also a fear of mine. This makes sense for those who run CO2 on timers vs pH controllers, the pH swings won't affect the fish because the chemical makeup of water itself isn't changed. This was a very good post!
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