Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
It isn't the pH swings.
It is the changes in mineral levels.
Since CO2 on/off does not change mineral levels the fish are fine with it. Generally you are aiming for a 1 unit change in pH, for example, 7.0 when it is off down to 6.0 when it is on. Fish are fine with this.