They do not care about pH, they do care about CO2 if one generalized and drew comparisons.

I can have a pH of 7.2 and a CO2 of 35 ppm and KH of 20.

Likewise I can have a KH of 1 and pH of 6.0 and a CO2 of 30ppm.
Plants generally will do better in the higher KH waters under high light.
Some plants, maybe 5-10 species will do better in the softer water, but it's less likely to be due to pH, and much more due to KH.

We can artifically change the pH with various organic acids easily, and leave the
KH the same. The same effect is noted for several species under this situation.
So KH is more likely the issue and the cause rather than pH.

KH is only part of the issue(CO2 is the other part), assuming pH drives everything is a poor assumption as well.


Regards,
Tom Barr