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Dosing Calcium and Magnesium in CaCO3 Equivalents


18 Dec 2016
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.



27 Aug 2018
Gloucestershire, UK
Secondary Macros Calcium and Magnesium.
Calcium sulphate is not very soluble better to use calcium chloride.

There is much discussion about Ca:Mg ratios. This appears to be based on the analysis of natural waters. In practice most UK water are very low in magnesium while USA water are by comparison rich in this element.

Do you know the analyses of your tap water? It may contain all the calcium you need (not much is required for plant growth). If your water is hard (high GH) cut it with rain water or RO water. If soft (low GH) then add your minerals. The ratios are not critcal as very thing is very dilute and can be reset with a weekly 50% water change. I assume that you are following an Estimated Index approach to plant growth.

You are on the right track with your calculations. Atomic Masses are just ratios. If you want to be accurate use a cheap digital balance from say eBay. For very tiny amounts such as trace element mixes scale up by 10 and then dilute the volume in solution by 10 and so on. Again its just ratios.

Good luck with your endeavours.


Expert/Global Moderator
11 Jul 2007
Chicago, USA
Totally agree with Oldguy. Worrying about Ca:Mg ratios is another waste of energy. Worry about keeping your tank clean with massive and frequent water changes and worry about CO2/flow/distribution, which are 1000 times more important than ratios.


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