Calcium requirements in water

> From: Hoa Nguyen <nguyenh at nosc_mil>
> Subject: Re: mixing powders & substrate vs. dissolved nutrients
> How much Calcium (mg/L) in the tap water do you consider 
> sufficient?

If you added 1 tsp (2 grams) of CaCO3 for 50 liters of water
you get 2 degrees KH & GH increase. That is equivalent to
40 mg/L of calcium carbonate. I think that is how calcium concentrations
are expressed. Since I don't have a periodic table, I don't
know what that is in terms of elemental calcium but the important
value is the measure supplied by your water utility analysis.

I never really got an answer in my quest for an optimal or
safe minimal value for Ca concentration but since a common GH
hardness target is 4-6, I figured 40 mg/L would be a safe