"Aqua" wrote in message
...
You can subtract Ca from GH (normally Ca+Mg)...

Sorry guys, How do you measure Ca? Do you have any test kit for Ca?

Yes, there are test kits for Ca, but they're not used much. Hagen makes one.
The way the Ca & Mg concentrations are almost always obtained is from a
water quality report. Your water utility will probably be glad to send you
one. Or it might even be published on the Web.
__
Alex
pcalex (at) hotpop.com