Do changes in pH due to CO2 affect fish?

[souffle]

Hi planted tankers,

I've read on this forum that pH swings due to CO2 changes are not harmful to fish (I'm looking into whether I should be turning off CO2 at night)

On the other hand, I've also read that adding CO2 to a tank is a good way to make conditions better for fish requiring low pH.

Surely these can't both be right? Anyone know which one is correct?

Thanks for your help!