I should also mention that sudden changes in pH is another one of the things that can probably cause damage. But I would also argue that if your discarded water has a pH that has gotten high or low enough for the sudden differential a 100% water change will result in to cause cellular damage, you've got far more serious, longer term water chemistry problems, and a a 100% water change is probably an advisable short term risk to get your system out of a dangerous zone.
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