# Do I need Mg ?



## PeteInEssex (Jul 29, 2006)

Mg testing
((17.86 x dGH) - (2.5 x Ca ppm)) / 4.1 = Mg ppm

Using this formula I get the follwoing - 

((17.86 x 13) - (2.5 x 95)) / 4.1 = 
(232 - 237.5) / 4.1 = -1.34

Hmmm - so I have -1.34 ppm Mg/l  

The dGH and Ca ppm were derived from the Nutrafin test kits and these results have been stable over several weeks.

Fortunately I do have a bag of MgSO4 - should I use it??

Thanks
Pete


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## Salt (Apr 5, 2005)

I'm confused as to why you would multiply your calcium reading by 2.5? I've never heard of doing anything like that. The Calcium reading should already be in a CaCO3 equivalence (the same equivalence as your GH reading). So you'd take 232 - 95 = 137 / 4.1 = 33 ppm, which I view as WAY WAY too high. It should be around 5, no higher than 10.


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## PeteInEssex (Jul 29, 2006)

Hi Salt - thanks for your reply.

I got the formula from this forum - 
http://www.aquaticplantcentral.com/forumapc/science-of-fertilizing/2427-faq-new-co2-charts-etc.html

I'm using Tom's EI dosing in my 100 (UK gallon) tank (approx 80 gallons water) -
1 1/5 tsp NO3 and 1/4 tsp KH2P04 every other day

1/2 tsp trace (every other day) containing- 
Boron 1.06%
Copper 0.23%
Iron 8.2%
Manganese 1.82%
Molybdenum 0.15%
Zinc 1.16%

50% water change weekly

Ca reads ~100ppm
K dH = ~6
G dh = ~13

Ph 6.5 - pressurised CO2
3 x 36w triton tubes (12 hrs/day)
2 x 150w metal halide 6,500k (9hrs/day)
Eheim 2329 wet/dry
Eheim 2217 into Aqua Medic 500 reactor
well planted tank
medium-high fish load

Thanks
Pete


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## Glouglou (Feb 21, 2006)

*Calculating MG*

Well Pete, I already discuss how to calcul MG in this post->
http://www.aquaticplantcentral.com/forumapc/science-of-fertilizing/3280-dosing-magnesium.html

I have the exact same formula that you have (I try to remember where I got that formula)

(GH (Ca + Mg) en ppm) - (2.5 x KH en ppm)
------------------------
4.1

anyway, no clear answer came from that post

To help I find that other assumption:



> Mg is only suplemented when Mg levels fall below 45 ppm. In this example, our GH (Total Hardness) equals 120 ppm and our KH (Carbonate Hardness) equals 80 ppm as calcium carbonate (CaCO3). Since GH equals the amount of Mg and Ca present, I can now simply subtract the KH (80 ppm) from the GH (120 ppm) and get a Mg concentration of 40 ppm


Somebody out there with chemist abilities should be able to help


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## PeteInEssex (Jul 29, 2006)

Now I remember why I always preferred physics to chemistry......

Pete


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## Salt (Apr 5, 2005)

I believe Edward's formulas are wrong, or at least misleading.

If you do a GH test, most all of them give you a reading in a calcium carbonate equivalent. 1 degree contains 17.86 ppm of calcium carbonate.

Calcium tests typically are the same as GH tests, except they use an inhibitor that prevents magnesium from showing up on the test. The readings are also in a calcium carbonate equivalence.

It looks to me like Edward believes that calcium tests give readings as *just calcium*, which I don't believe is correct (unless I'm missing something... I mean no offence to Edward, that's just what it looks like to me in the post linked above). For example, Lamotte's 3609 calcium test kit gives readings as CaCO3. (http://www.lamotte.com/pages/common/pdf/instruct/3609.pdf)

GH ppm as CaCO3 minus calcium ppm as CaCO3 would equal magnesium ppm in a CaCO3 equivalence. In order to translate that to just magnesium cations, you would divide it by 4.11796.

The 2.5 is for calcium readings only. If you do a calcium test and get 95 ppm as CaCO3, what that means is that if the only compound you have in the water is CaCO3, then you have 95 ppm of CaCO3. To get just a calcium reading from this, you *divide* by 2.497303 to get ppm of calcium. If all you're after is magnesium ppm, there's no need to do this.


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## PeteInEssex (Jul 29, 2006)

Many thanks for your detailed analysis Salt.
So it looks like I have around 30ppm Mg, which is more realistic than -1.34ppm Mg !

I've just done a test on my tap water - my Nutrafin test kit shows - 
Ca2+ mg/L = 100 ppm
GH = 240 or 13.44 dh°

The instructions say
To determine the ratio between Ca and Mg
Calcium mg/L (Ca2+) x 2.5 = Calcium Hardness mg/L (CaCO3)
GH mg/L - Calcium Hardness mg/L = Mg Hardness (CaCO3)
Mg Hardness (CaCO3) / 4.1 = Magnesium mg/L

so,
Calcium mg/L (Ca2+) x 2.5 = 250 mg/L (CaCO3)
240 - 250 = -10 Mg Hardness (CaCO3)
-10/4.1 = -2.44 mg/L of Magnesium

So it appears that Nutrafin have got their calculations wrong (maybe that's where Edward got his examples from).

Salt, do you agree that what Nutrfin say is wrong? if so I'll politely query it with them.

Many thanks again Salt.

Best wishes
Pete


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## Salt (Apr 5, 2005)

The kit should say what equivalence its measurements are in (Ca, or CaCO3). I am not familair with the Nutrafin kit, so I have no idea. However, if the *kit's* instructions are telling you to multiply its calcium readings by 2.5, then it's very possible the kit gives readings as calcium, not calcium carbonate.

If that's the case, then the formulas are correct (both the instuctions and Edward's formulas). It would also be reasonable to assume that you have very little magnesium in your tank since the GH and Ca readings are about the same.


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## PeteInEssex (Jul 29, 2006)

Here's the word for word instructions from the Nutrafin tests for Ca - 
(NB - I used 5 drops of reagent 3)

CALCIUM ANALYSIS VERSUS GENERAL HARDNESS ANALYSIS

Ca2+ mg/L (ppm)
Multiply the total number of drops of reagent #3 used by 20 to determine the Calcium concentration.
To determine the ratio between Calcium & Magnesium
1. Calcium mg/L (Ca2+) (multiply by) x2.5 = Calcium Hardness mg/L (CaCO3)
2. General Hardness mg/L (CaCO3) (minus) - Calcium Hardness mg/L (CaCO3) = Magnesium Hardness (CaCO3)
3. Magnesium Hardness (CaCO3) (divided by) / 4.1 = Magnesium mg/L Mg2+ 


Pete


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## Edward (May 25, 2004)

To make this more complicated is the fact that the best Mg concentration is the one where you dose MgSO4 daily and still test close to zero ppm. See the PPS dosing table for details.


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