It's important that the pH stays stable for any fish species, which is why that for 95% of fish you should absolutely NOT attampt to fiddle with the pH.
Most fish will happily adapt to whatever pH your water naturally is as long as extremes are avoided, and Dwarf Puffers are certanly one of them
Our Dwarfs have been livining in pH7.8 for two years now.
I agree, most of my fish (including my dwarf puffers) seem to thrive in my pH (around neutral) Our goo obo gudgeons came from California, with a pH of 8.2, and they adapted well. If you are getting your dwarf puffers from your area, then chances are they are already living in water with the same pH as you water
EDIT: Does co2 injection alter the pH? I've never actually looked into it.
yes it generally lowers pH. But also the pH will fluctuate during the day as the plants don't use CO2 at night, so the readings first thing in the morning and last thing before lights go out will be different.
pH at the moment is 6.6, but I expect this to stabalise a bit more when I have a full tank of plants.