Thusly Elaine T Spake Unto All:

Thanks - that makes sense. Basically you're saying that bicarbonate
from calcium carbonate only reaches its 8.3 pKa in natural waters where
the amount of limestone is huge and the water has years of slow contact,
right?

Pretty much, although in most cases you'll never actually reach 8.3.
Even in areas where the bedrock is limestone, natural surface waters
usually have a pH of 7.5 - 7.7-ish.

I actualy don't have too much trouble with my liqid kits getting
reproducible measurements within a given day in my hardwater non-CO2
tanks (I've tried this), but you're correct that I don't know the accuracy.

The liquid kits usually aren't bad, and for normal aquarium use you
don't need to know the pH with greater accuracy than 0.5 - 1 unit; pH
is simply not that critical unless you have a specialty aquarium (e.g.
high-tech plant tank or maintain a pH below 6 or above 9). The pH
papers and multisticks, however, are atrocious. Maybe they're OK when
they're fresh from the factory, but after a year or so on the shelf
they're usually so wildly off that one IME is better off without them.