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Hard water, pH

Steve wrote: 

>OB aquatic plant content: So is pH really the problem with "hard"
>not the high concentration of Ca & Mg cations? Do carbonates have any
>undesirable effect aside from the pH? Ole wrote to me privately about
>aquarists using ion replacement (DI?) water and then treating it with
>acid to remove the carbonates. Wouldn't it be just as effective to
>merely treat the water with acid to remove the carbonates and leave the
>Ca & Mg in solution? (or as precipitate)

I believe he meant water that has been run through an ion exchange
column.  A water softener is an example of a cation-exchange column,
where Ca and Mg are exchanged for Na, or K.  Anion-exchange columns also

Sure, you could treat water with a strong acid to remove the carbonates.
However, you would have to drop the pH at or below 6.35 (pK1) to convert
at least half of the carbonates to CO2.  Then, if you wanted a higher
pH, you would have to titrate the water back up, using a base.  Not me.

I am usually suspicious of reports from freshwater aquarists complaining
they can't get their pH below 8.0, despite CO2 injections, peat
treatments, etc.  Sea water, with an alkalinity of 2.5 meq/l, a combined
Ca and Mg concentration of 1,750 ppm (!) and a total dissolved solids of
36,000 ppm, has a pH of "only" 8.2, and that can easily be brought down
to 7.8 or less with simple CO2 injection.  While I certainly acknowledge
the water quality problems of some folks, I suspect many of the
"problems" reported here are due to inaccurate test kits, not poor