Ok thanks for that somewhatshocked. Have done some reading and think I get it(more or less):
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.