OK, I went back and did a search. This message was from Rod Farlee
who IS a chemist. this is what he said on 6/16/1999
Ingrid

"But this "pH pill" is not the "one size fits all", "toss it in and
forget it" remedy that the ******** suggests it is.

It certainly works, but with some caveats. It raises GH (general
hardness, calcium) more than calcium carbonate (limestone) alone
does. That's because it contains calcium sulfate (gypsum), which
is much more soluble than calcium carbonate, and which raises
GH but not KH (alkalinity, bicarbonate).

So unless one has very soft, slightly alkaline water to begin with,
care should be taken. GH and KH should both be monitored.
If GH is getting too high, take it out, and add baking soda if needed
to raise KH. Or limit the amount to less than 1 lb/1000 gallons, to
ensure GH does not continue to rise.

If much larger amounts of plaster of Paris are added, it just won't
work. Gypsum is 170 times more soluble than calcite. Gypsum
will continue to dissolve, and push GH "off the scale". Through the
common ion effect (calcium), this will suppress dissolution of the
calcite (calcium carbonate), so KH will remain low. If a very large
amount were used, GH and pH would go up, and KH would actually
go down, and the pond might get cloudy with calcite precipitation.
That would be a Bad Thing: it's the next step on the road to an
alkali lake. Gotta watch for that.

Happily, it takes some weeks for the "pH pill" to dissolve anyway,
so these gradual changes can be monitored. And the effects of an
"overdose" are ameliorated if one's water contains a substantial
magnesium component in it's GH (the average eastern or
midwestern river does, Ca/Mg ~ 4). (Reasons: MgCO3 is much
more soluble than CaCO3, and it interferes with calcite cystallization
so allows supersaturation to occur. This reduces high GH driving
KH down.) Finally, if one receives regular rainfall, one might never
see these overdose effects. But the potential is there, and I treaded
close to it before I understood what was happening."