NetMax wrote:
Personally I think that it's the osmotic
pressure (water hardness) which has more
bearing than the pH. [lots deleted]

I should point out that there's an assumption behind what NetMax says
when he equates osmotic pressure and hardness. Namely, his assumption
is that the majority of the solutes contributing to osmotic pressure
also contribute to water hardness, GH.

To demonstrate, here's an (absurd) comparison:
Water A:
2M in NaCl
5.0 ppm in Mg++

Water B:
0M in NaCl
5.0 ppm in Mg++

Both these waters will have the same GH on a hobbyist-type GH test kit.
There will, however, be a pretty big difference in osmotic pressure.

I'm certainly not saying that NetMax's assumption is invalid. It may
work fine for even a large majority of aquarists. The only way to know
if it applies in a particular case is to know, as intimately as
possible, the source water and what has been put into it.

I would further submit that a truer way to get a handle on osmotic
pressure would be to measure total dissolved solids (TDS) or to measure
conductivity.

Regards,
Trapper