Dosing Calcium And Magnesium In Caco3 Equivalents

Harris Tiu

New Member
Mar 16, 2019
Hi Folks,

I am now learning about how to dose the Secondary Macros Calcium and Magnesium.
I've read its 3:1 ratio between Ca:Mg. Also, im supposed to dose 1dGH worth after my weekly water change.
So then its dosing 15ppm for Calcium and 5ppm for Magnesium.

I am supposed to dose in CaCO3 equivalents right?

My calculations for Calcium are as follows:
Calcium dose 15ppm = 15mg/L while Tank is 400L.
15mg/l x 400L = 6000mg = 6g of Calcium. I want to dose 6g of Ca.
I'm using CaSO4.2H20 which is 22.5% Calcium.
Since I need to dose 6g of Calcium, then want to dose 26.67g of CaSO4.2H2O. Thats about 1.6 Tbsp.

However, if I were to dose in pure ions, I would have to divide 15ppm of Calcium by 2.5.
15ppm Ca in CaCO3 equivalents will become 6ppm pure ions of Calcium.
And then I do the above calculations again.



Super Moderator
Staff member
Lifetime Member
Jun 20, 2016
agree, there is a discussion and confusion behind it , but nowadays the ratios given above are meant to mean mass ratios of the ions... irrespective of the salts used. So your first calculation goes in the right direction. 15 mg/L Ca and 5 mg/L Mg would translate into 3.2 °dGH but really when you know the exact Ca and Mg conc., GH is of less importance.