Magnesium Test Kit - The Planted Tank Forum
Old 03-07-2016, 05:08 PM Thread Starter
Algae Grower

Join Date: Feb 2012
Location: Novi
Posts: 9
Magnesium Test Kit

Hello All,

I'm getting back into my planted tank and wanted to know if the Salifert Mg test kit was ok to use on freshwater?
Or if anyone has a better way of testing for Mg in their water?

cicrush13 is offline

Old 03-07-2016, 08:55 PM
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There is no need to test for Mg. When you add it you know it’s there.
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Edward is offline
Old 03-07-2016, 11:41 PM
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There is a fresh water calcium test (salt water ones won't work unless labeled for both).
Test GH and convert (if needed) to ppm.
Test Ca in ppm.

Use this formula:

((GH in ppm) - (2.5 x Ca in ppm))/ 4.1 = Mg in ppm
Diana is offline

Old 03-08-2016, 12:13 AM
Algae Grower

Join Date: Sep 2013
Location: Outside Boston
Posts: 73
Quote:
Originally Posted by Diana View Post
There is a fresh water calcium test (salt water ones won't work unless labeled for both).
Test GH and convert (if needed) to ppm.
Test Ca in ppm.

Use this formula:

((GH in ppm) - (2.5 x Ca in ppm))/ 4.1 = Mg in ppm
I am sorry to hijack but I was hoping to get some clarification on this formula. I use the LaMotte GH test kit which tests as CaCO3. I assume this formula does not hold true for CaCO3 right? Because my test values:

GH: 80 ppm as CaCO3
Ca: 64 ppm as CaCO3

(80 - 160) / 4.1 = -19.5

I have been using the difference between the two values for magnesium as CaCO3. What would be the proper way to calculate my magnesium?
duderino is offline
Old 03-08-2016, 12:27 AM
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CaCO3 ppm equivalent to Ca and Mg ppm and dGH

Quote:
Originally Posted by duderino View Post
I am sorry to hijack but I was hoping to get some clarification on this formula. I use the LaMotte GH test kit which tests as CaCO3. I assume this formula does not hold true for CaCO3 right? Because my test values:

GH: 80 ppm as CaCO3
Ca: 64 ppm as CaCO3

(80 - 160) / 4.1 = -19.5

I have been using the difference between the two values for magnesium as CaCO3. What would be the proper way to calculate my magnesium?
You are correct; this formula
(ppm GH - (2.5 x Ca ppm)) /4.1 = Mg ppm
is not for CaCO3 equivalent.

Example:
GH 80 ppm as CaCO3
Ca 64 ppm as CaCO3
...
Ca ppm = Ca as CaCO3 / 2.5
Ca ppm = 64 / 2.5
Ca ppm = 25.6

Mg ppm = (ppm GH as CaCO3 – ppm Ca as CaCO3) / 4.1
Mg ppm = (80 – 64) / 4.1
Mg ppm = 3.9
...
Ca dGH = Ca ppm / 7.144
Ca dGH = 25.6 / 7.144
Ca dGH = 3.58

Mg dGH = Mg ppm / 4.356
Mg dGH = 3.9 / 4.356
Mg dGH = 0.9

Total dGH = Ca dGH + Mg dGH
Total dGH = 3.58 + 0.9
Total dGH = 4.48
...

Last edited by Edward; 03-08-2016 at 05:09 PM. Reason: .
Edward is offline
Old 03-08-2016, 03:11 PM
Algae Grower

Join Date: Sep 2013
Location: Outside Boston
Posts: 73
Awesome! Thank you very much! The instructions in the kit tell me how to convert ppm into grains per gallon but I have been wondering how to convert into a more commonly used measurement.

I typically take these values and plug them into the Zorfox calculator to get my hardness. Have I been doing it incorrectly by putting in 64ppm as Ca and 16ppm as Mg as CaCO3 equivalent and getting 4.48 dKH? The other value in that calculation is that my iron tests at .2 ppm. I do not have a test for Manganese.

BTW...I also agree that there is no need to test for this but when I had a new growth issue and had several responses that my problems were toxicities or different deficiencies it was nice to be able to know for sure what certain levels were in order to make corrections.

Last edited by duderino; 03-08-2016 at 10:03 PM. Reason: correction
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Old 03-08-2016, 04:50 PM
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CaCO3 ppm equivalent to Ca and Mg ppm and dGH

Quote:
Originally Posted by duderino View Post
... I typically take these values and plug them into the Zorfox calculator to get my hardness. ...
...
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Last edited by Edward; 03-08-2016 at 05:10 PM. Reason: .
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