# Is it possible ?



## BigFoot (Jan 3, 2005)

Is it possible to have negative number when testing for Mg. ?


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## Rolo (May 12, 2004)

The lowest you can have is nothing, zero. Is this from a test kit or your own calculations with Ca + GH kit?


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## niko (Jan 28, 2004)

If you play with the numbers for a while you will see that a small change makes a big difference in the Ca:Mg ratio.

Use this:
http://www.deepforestaquatics.com/aqua/CaMgRatio.html
and enter GH=4 and Ca=20. Then enter GH=4 and Ca=30. The Mg becomes negative with only 10 ppm difference.

If I read the "FAQ" right (http://www.aquaticplantcentral.com/forumapc/showthread.php?t=2427) then if the water has only Ca then 1 dGH is provided by 7 ppm Ca. So in the example above with GH=4 if the Mg=0 then the Ca cannot be more than 7 x 4 = 28 ppm. But it is not unusual to get test results of say Ca=40 and GH=4 maybe because the resolution of the test kit I use is 10 ppm.

I don't know if the formula is not entirely correct or what. We left it as it is, showing negative numbers, because it seemed to give you an idea how far is the spread between Ca and Mg when the Ca was way more than the Mg.

To me personally the bottom line is what Edward has been preaching lately - one must have Ca and Mg in a ratio that is close enough. Ratious like 4:1 or 10:1 are not written in stone but a 100:1 ratio is most likely way off. And surely one should not have a negative number 

--Nikolay


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## BigFoot (Jan 3, 2005)

Oh i didnt know that the 4.1 was a ratio. Here are my numbers gh = 220 and ca = 80 both are ppms. What is my Mg and how do i check from now on. Thanx in advance.


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## shalu (Oct 1, 2004)

You have Hagen calcium kit and it gives number in Ca, not CaCO3, so Mg would be:

Mg = (220 - 80*2.5)/4.1 = 5ppm

But Ca has a resolution of 10 ppm, I guess GH the same resolution? 
The Mg calculation can be up to 6 ppm off, I would say it is between 2.5-11 ppm( assuming 220 GH really means 210-220ppm, 80ppm Ca really means 70-80ppm)


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## niko (Jan 28, 2004)

BigFoot,

The formula that I referred you to doesn't use ppm but dGH. 1dGH is 17.9 ppm, so:

220/17.9 = 12.3 dGH

Then if you use GH=12 dGH and Ca=80 ppm you get:
Mg=3.5 ppm
Ca:Mg = 23:1

I agree with shalu, the tests are not very precise and that is why I believe that some Mg and some Ca should be present but there is no need to chase precise ratios.

--Nikolay


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## BigFoot (Jan 3, 2005)

I am not looking to get a certain number on this mineral butt i am looking for a good range to be in. I have a high light tank so from i am told they nutrient suckers big time. Niko as far as the 5 ppm that u gave me that is close to the number i had (4.87). Aloso niko i want to thank you for calculator i will use in the future. Shalu i do hagen test kits both measure in 20 ppm intervals i get what u mean that have certain range to them and that they dont show dead on what the tank is. This what i use to figure out the Mg. 



(gh in ppm) - (2.5 x ca ppm)
________________________________
4.1


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## Edward (May 25, 2004)

Hi BigFoot

If you use 10ml instead of the 5 ml then your Ca resolution is 10 ppm, not 20 ppm. With Hagen Ca test kit.


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## shalu (Oct 1, 2004)

Edward said:


> Hi BigFoot
> 
> If you use 10ml instead of the 5 ml then your Ca resolution is 10 ppm, not 20 ppm. With Hagen Ca test kit.


What if one uses 20ml, 50ml? Would it still work, just uses more reagent?


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## Edward (May 25, 2004)

shalu said:


> What if one uses 20ml, 50ml? Would it still work, just uses more reagent?


Hi Shalu

Yes, we can use more water. With the Hagen Ca test kit we still want to put 7 drops of #1 and 3 drops of #2.

Solution #3:
1 drop in 5 ml = 20 ppm 
1 drop in 10 ml = 10 ppm
1 drop in 20 ml = 5 ppm
&#8230; and so on &#8230;

The limit is the visibility of the color change from pink to blue.

Edward


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## BigFoot (Jan 3, 2005)

HI 


Edward all i am looking for here is if i did the math right i was shooting for 5 to 10 ppm of Mg. If so with numbers provide i got 5 ppm now at least that is what i come up with.


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