1 - 7 of 7 Posts

#### cicrush13

·
##### Registered
Joined
·
9 Posts
Discussion Starter · ·
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?

#### Edward

·
##### Registered
Joined
·
1,505 Posts
There is no need to test for Mg. When you add it you know it’s there.
.

#### Diana

·
##### Registered
Joined
·
11,721 Posts
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

#### duderino

·
##### Registered
Joined
·
75 Posts
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?

#### Edward

·
##### Registered
Joined
·
1,505 Posts
CaCO3 ppm equivalent to Ca and Mg ppm and dGH

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
...

#### duderino

·
##### Registered
Joined
·
75 Posts
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.

#### Edward

·
##### Registered
Joined
·
1,505 Posts
CaCO3 ppm equivalent to Ca and Mg ppm and dGH

... I typically take these values and plug them into the Zorfox calculator to get my hardness. ...
...

#### Attachments

• 17.5 KB Views: 165
• 19.9 KB Views: 150
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.