How to read the Ca/Mg reading of GH - The Planted Tank Forum
 
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post #1 of 7 (permalink) Old 01-17-2005, 12:43 AM Thread Starter
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How to read the Ca/Mg reading of GH

I just bought a Calcium testkit to find out how much Mg I actually have.

Readings are:

GH - 7 dGH x 17.9 = 125ppm
Ca - 60ppm

so Mg must be ~65ppm

I have no real clue if this is a lot of Mg or not, but what is the expected range in PPM for Mg?

Another question... while using the Ca testkit, it says the color will go from pink to purple to finally blue when seeing how many drops it takes to reach your Ca level. the chart shows this very dark shade of blue that I never reach. It does however change to pink after the first drop, purple after the second, and then a lighter blue on the third drop (20ppm per drop). Should I assume the sign of Blue at all means it is done going through the color-changing process? I am using the Aqua Pharmaceuticals brand.

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post #2 of 7 (permalink) Old 01-17-2005, 03:49 AM
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Are you using the Ca kit for SALTWATER? That's what I have because I can't find one for freshwater. With the one I have, the chart is useless because the scale would be different for freshwater. I did an experiment, added lots of table salt and the reading is much higher, which means, the chart reading on freshwater is lower than actual value.


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post #3 of 7 (permalink) Old 01-17-2005, 05:00 AM Thread Starter
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ya, I think it is for Saltwater. any idea how to test Ca for FW then? maybe there is a way to convert the reading over on the SW testkits.. more/less of the first 10 drops maybe (the 1st of the 2 bottles)?

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post #4 of 7 (permalink) Old 01-17-2005, 05:21 AM
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I had an idea that I could add table salt(no calcium) to tap water until I get equivalent sea water TDS, unfortunately, my TDS meter does not go that high. So I could only do a boundary test --- tested saturated salt water. True reading should be somewhere in between.


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post #5 of 7 (permalink) Old 01-17-2005, 05:38 AM
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Or maybe even buy some reef salt and use that. Another thing to consider is if your kit measures the 'low' Ca levels we have in freshwater. Saltwater has much, much higher Ca. Once you get past that issue then consider this...

Quote:
GH - 7 dGH x 17.9 = 125ppm
Ca - 60ppm

so Mg must be ~65ppm
Unfortunately it is more complicated. We can't substract the Ca ppm from the GH ppm and get Mg ppm. The units must be considered. GH in ppm is expressed as CaCO3.

60ppm Ca x (100.09MM CaCO3/40.08MM Ca) = 150ppm CaCO3 (greatly truncated this calculation in the effort of being simple)

This means 60ppm of Ca is equalivalent to 150ppm GH. Right off the bat this can't be since your GH is only 125ppm. Anyway, when you get the right Ca measurment - substract your Ca-to-GH equalivalent (in this example it's 150ppm) from your actual GH. This answer I'm calling "X"...

Xppm GH = Xppm CaCO3

Xppm CaCO3 x (24.31MM Mg/100.09MM CaCO3) = ? ppm Mg

I'm not perfect, so please point out mistakes if found.
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post #6 of 7 (permalink) Old 01-18-2005, 01:39 AM Thread Starter
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sounds fun and complicated

think it would be a safe bet to just add a little more Mg each water change just in case there isnt enough? the plants are growing fine and I just added 2-3x the wpg I used to have, so I figure I was ok, but just curious as to my Mg levels. rather than going to the trouble of adding reef salt, etc, and I have a current GH of 7, what is a safe amount of Mg (I think in the form of MgSO4) to add to my tank each water change? It is for a 50g tank (~40g's of water).

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post #7 of 7 (permalink) Old 01-18-2005, 03:49 PM
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Send a message via AIM to IUnknown
http://lamotte.com/pages/common/pdf/...t/4824drlt.pdf

Lamotte makes a good kit to measure mg and ca separately.
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