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  1. #1
    Senior Member SloHand's Avatar
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    Determining Magnesium Levels

    At the outset, I'd like to say that I am using a Hagen test kit ... It's what is available here. That said .... On the test kit there is a "Calcium analysis versus General hardness analysis" and a "ratio between Calcium & magnesium" What I'm trying to do is determine my magnesium levels 'cause I want to know whether or not to dose with 'Tech M'. I used to dose automaticly once a week and I don't want to keep just adding stuff blindly without knowing if it's needed or not. BTW I do drip kalk as top-off.

    Here are my test results this morning at 'lights-on'

    Temp = 78.5F
    pH = 8.0
    Nitrite = 0
    Nitrate - <5
    Phosphate = 0
    Calcium = 360 (with the Hagen kit)
    KH = 9 dkh (161 mg/L)


    So Hagens formula for determining Magnesium is;

    1) Calcium mg/L X (multiplied by) 2.5 = Calcium Hardness mg/L
    2) General Hardness mg/L - (minus) Calcium Hardness mg/L = Magnesium Hardness
    3) Magnesium Hardness / (divided by) 4.1 = Magnesium mg/L

    I don't understand the results of these calculations? Help. What does it mean?

  2. #2
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    General hardness is not the same as dkh (carbonate hardness). GH (general hardness) measures calcium and magnesium while dKh measures alkalinity or buffering capacity. Make sure you measure GH (which should be very high for salt water). Then, the formula will make sense and not give negative values for magnesium.

    These references might give some useful background:

    http://www.thekrib.com/Plants/CO2/ha...arryfrank.html

    http://www.reefcentral.com/forums/sh...hreadid=128878
    Nick

  3. #3
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    SloHand,I too was using Hagen's calcium test kit..like you what was available.I dosed with kalk,used kent's tech A & B in an attemp to raise calcium levels in order to help with coraline growth.My readings never changed from 360ppm like yours.I have since changed to Red Sea's test kit...mind you I also tried Tropic Marin's Bio-Calcium supplement(Marinescape's advice,great store!) and according to this kit Im now at 450ppm.Just my 2 cents but I find it strange that my hagen tests were the same as yours...

  4. #4
    Senior Member SloHand's Avatar
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    Thanks for those links nickb. and your right about the GH being high ... I was at 51 drops and still didn't get a change in color to indicate a level (again a lousy Hagen kit) I'll try again and re try the calculation.

    Fish_business ... that is strange that our readings would be identical. I've had the kit for three weeks now testing every third day or so and same as you ... no change to that 360 reading and you'd think there would have been a change given that I've been using kalk paste on and off during that same period to fight off a bloom of Aiptasia.

  5. #5
    Senior Member reefmutt's Avatar
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    SloHand you will get nowhere with that hagen test kit and you will never be able to determine your mag level because you will never get an accuratre reading with the hagen kit.
    Don't waste your time or money- get some decent test kits-Salifert calcium, magnesium and dkh- then, there will be no guess work or formulas to use and you will have your answers.
    Matt.

    Old system torn down to make a playroom.. planning a 62x42x28 high

  6. #6
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    Reefmutt's adivce is good: get a magnesium test if you want magnesium.

    Just for interest, magnesium levels in reef tanks tend to be in the 1200-1300 range. If you work backward using the formula, the GH (general hardness) whouls be somewhere around 6000 mg/l. How many dropss would you need to get that reading?
    Nick

  7. #7
    Senior Member SloHand's Avatar
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    Lots o drops ... plop plop plop plop ... hehehehe yup did the math before trying that idea. No doubt eh reefmutt, using hagens test kits is an execise in frustration. New test kits are at the top of the shopping list ... as soon as the money tree bears new fruit

    Good advise all around, thanks

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