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Re: Hardness, Calcium, Magnesium

It has been more than a few years since I had Chemistry but I did fairly
well in it.  I now find myself forced to become somewhat proficient at it
again as I have come to realize that it is a must if I want to have a
successful planted aquarium.  Anyway, the messages below (pieced together
from two messages) has left me a bit confused and I would appreciate some
help understanding the reasoning behind the portion of the calculation
pertaining to Mg.

First of all, the atomic weights of the elements involved
C - 12.011, O - 15.999, Mg - 24.035, Ca - 40.08

MgCO3 - 24.035 + 12.011 + 3 * 15.999 = 84.043
MgCO3 is 24.035/84.043 or 28.6% Mg by weight

CaCO3 - 40.08 + 12.011 + 3 * 15.9999 = 100.088
CaCO3 is 40.08/100.088 or 40.0% Ca by weight

Following the calculation for Ca++ concentration is pretty straightforward
but I do not follow the rational behind the Mg++ calculation.  Why is the
atomic mass of CaCO3 used instead of the atomic mass of MgCO3 as the
denominator of the Mg calculation?  I would guess that the source of my
problem relates to the nature of the test and the resultant measurement
being in terms of CaCO3.

BTW - Although I have had fish with a few plants off and on for the past 25+
years, I'm fairly new to this planted tank stuff.  I used to think the look
of a more natural environment (a few plants and a couple of rocks) would
make them more content, after all happier = healthier.  I never dreamed of
attempting to recreate a truly natural environment for them as I thought it
would be way too difficult to try.  Like most others in the fish hobby I did
not realize the true impact of a well planted tank on the well being of the
fish and how truly beautiful a well planted and maintained tank could be.
Anyway, as you all know there is a lot to learn and I want to thank all of
you for providing so much help.  I have read through many of the messages
and looked at numerous websites.  It has provided a wealth of knowledge on
the subject that would have taken years to assimilate and probably at
considerable cost (mistakes can be expensive).  Thank you all so very much !
! !

Roger -
>Umm.  Buy low, sell high.
Although probably not one of the more eloquent offerings of wisdom on this
forum, it is surely the most practical.  Have you perfected this too or are
you just good with plants and fish?

Charles Kuehnl

>From APD V4 #950 and #951:

>Date: Sun, 08 Apr 2001 12:35:22 -0600
>From: "Roger S. Miller" <rgrmill at rt66_com>
>Subject: Re:  Hardness, Calcium, Magnesium

>John Fitch and Roger Miller wrote:

>Because I have been adding Calcium and Magnesium without really knowing how
>quickly they are (or aren't) being taken up by the plants, I bought a HACH
>hardness and calcium test kit, which provides magnesium by difference.  I
>would appreciate some help in interepreting the results:

>Hardness = 140 mg/l.
>Calcium = 80 mg/l.
>By difference, Magnesium = 60 mg/l.

>I believe these numbers are all in terms of CaCO3.  Am I correct in then
>multiplying the Calcium reading by 40/100 or 0.4 to obtain the Ca++
>concentration and the Magnesium reading by 24/100 or 0.24 to obtain the
>concentration?  This would mean:

>Ca++ = 32 mg/l
>Mg++ = 14 mg/l

>You're correct.

>Looks fine to me.

>Roger Miller