Calcium, magnesium and GH question - The Planted Tank Forum
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post #1 of 15 (permalink) Old 06-26-2020, 09:58 PM Thread Starter
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Ive been trying to find my magnesium levels in my planted tank by subtracting GH minus calcium. I tested my GH which is at 5, calcium is reading at 20ppm or below using the nutrafin calcium test kit. I used a converter to change 5 GH to ppm and it comes to 83ppm, so 83ppm - 20ppm calcium would be 63 ppm magnesium, at first I thought this was right but it doesn’t make sense.

I used the Rotala butterfly calculator, to measure 8ppm of magnesium and 20ppm of calcium in one gallon, the 20ppm of calcium in one gallon brings the GH to 2.8, and the 8 ppm of magnesium changes it to 1.8, so together in one gallon is 4.6 gh, this is by having 20ppm of calcium and 8 ppm of magnesium, total of 28ppm in 4.6 GH. So why when I used a calculator online to turn 4.6 gh to ppm it says 76 ppm? This is very confusing to me.
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post #2 of 15 (permalink) Old 06-26-2020, 10:11 PM
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How do I test for Mg?
First test for GH and Ca, then calculate Mg.
Mg testing
((17.86 x dGH) - (2.5 x Ca ppm)) / 4.1 = Mg ppm
(ppm dGH - (2.5 x Ca ppm)) /4.1 = Mg ppm

((17.86 x 5dGH) - (2.5 x 20)) / 4.1 = Mg ppm
89.3 - 50 = 39.3 / 4.1 = 9.59 ppm Mg
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post #3 of 15 (permalink) Old 06-26-2020, 10:27 PM Thread Starter
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Quote:
Originally Posted by Immortal1 View Post
How do I test for Mg?
First test for GH and Ca, then calculate Mg.
Mg testing
((17.86 x dGH) - (2.5 x Ca ppm)) / 4.1 = Mg ppm
(ppm dGH - (2.5 x Ca ppm)) /4.1 = Mg ppm

((17.86 x 5dGH) - (2.5 x 20)) / 4.1 = Mg ppm
89.3 - 50 = 39.3 / 4.1 = 9.59 ppm Mg
Ok this makes way more sense thanks. I have another question though, which I know will sound stupid but I prefer that than to mess up, where does the 17.86 and 2.5 come from? I’m assuming I will always have to multiply my GH by 17.86 and my calcium by 2.5, but where are these two numbers from?
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post #4 of 15 (permalink) Old 06-27-2020, 01:55 AM
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PPS Pro web site
LOT of good info on this site
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post #5 of 15 (permalink) Old 06-27-2020, 02:10 AM Thread Starter
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Damn that’s a great site thanks &#x1f44d;
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post #6 of 15 (permalink) Old 06-27-2020, 02:25 AM
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More great info
https://www.advancedplantedtank.com/


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post #7 of 15 (permalink) Old 07-19-2020, 09:24 PM Thread Starter
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I read a lot on that site. But now I’m having trouble keeping my tds at lower ppm. My tds Is 233 ppm, my gh is 4. Phosphates around 3, nitrates 18, I’m not sure on my potassium levels since is hard to test for that but with everything I don’t understand why my tds is this high, I use ro di water and mineralize 2 times a week using the seachem trace. I also don’t dose much iron. Non of these seem possible to raise the Tds to 230 Ppm. When I test my ro di water it reads 0 ppm so I think my meter is working good.
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post #8 of 15 (permalink) Old 07-19-2020, 10:57 PM
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Reading 0 is not a indicator that meter is calibrated. You need to buy or make your own calibration solution. 0 conductivity is 0 conductivity regardless of calibration. You canít measure 0.
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post #9 of 15 (permalink) Old 07-19-2020, 11:10 PM
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Do you use tap water or RO?

If you use tap water, it can be additives from the source.

If not, it could be carbonates (KH), or potassium.

Whoops - you use RO. It has to be the above. What is your TDS after adding your booster?
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post #10 of 15 (permalink) Old 07-20-2020, 11:33 PM Thread Starter
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Yeah I use ro di water, and my KH is usually 0, I try to keep it at 1 or 2 but sometimes I give up cause it keeps going down to 0 and I know most plants are ok with that and even do better. I’ll prolly just buy a new tds meter, this is a cheap one I got for like 15 dollars and I don’t have anything to calibrate it anymore. Might as well just get a better one and it will come with calibration solution.
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post #11 of 15 (permalink) Old 07-23-2020, 04:40 AM
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Quote:
Originally Posted by Alien87 View Post
I read a lot on that site. But now I’m having trouble keeping my tds at lower ppm. My tds Is 233 ppm, my gh is 4. Phosphates around 3, nitrates 18, I’m not sure on my potassium levels since is hard to test for that but with everything I don’t understand why my tds is this high, I use ro di water and mineralize 2 times a week using the seachem trace. I also don’t dose much iron. Non of these seem possible to raise the Tds to 230 Ppm. When I test my ro di water it reads 0 ppm so I think my meter is working good.
Curious how you are so precisely measuring nitrate.

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post #12 of 15 (permalink) Old 07-23-2020, 01:02 PM Thread Starter
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With api test kit, when I said 18 nitrates, it’s an estimate judging by color, it’s not exactly 18 ppm. But a bit more or less wont make a difference. I’m gonna buy the HM tds meter and see what readings that gives me for tds. The one I have now don’t even have the option to be calibrated so it’s prolly not accurate.
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post #13 of 15 (permalink) Old 07-25-2020, 01:43 AM
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Quote:
Originally Posted by Alien87 View Post
With api test kit, when I said 18 nitrates, it’s an estimate judging by color, it’s not exactly 18 ppm. But a bit more or less wont make a difference. I’m gonna buy the HM tds meter and see what readings that gives me for tds. The one I have now don’t even have the option to be calibrated so it’s prolly not accurate.
In my experience, most of the cheapo meters out there aren’t very accurate. They are useful for relative measurements though. That being said, your meter is probably fine. I wouldn’t stress about what all is making up the total ppm.

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post #14 of 15 (permalink) Old 07-25-2020, 02:34 AM
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I’m gonna buy the HM tds meter and see what readings that gives me for tds.
When you buy a new TDS meter, also buy some calibration solution at the same time. The meters say they are factory calibrated, but I've recently bought new meters (like you, to replace cheap ones that I didn't have confidence with!) and they were actually quite off.

Get a calibration solution that is close (maybe just above) the value that you are going to be measuring. 342 ppm is common.

When choosing a new meter, get one that is temperature compensated for more accuracy. I've recently bought the new HM Digital AP-1 which seems pretty good and is not expensive (even with shipping Amazon US to Singapore). Easy to calibrate and seems consistent with readings.

One other thing I recently learned / realised that I think is useful to understand: there is no such things as a meter that measures TDS. There are only meters that measure conductivity (EC) which have been programmed to multiply the measurement by a given factor to give a "TDS" readout. The factor used varies between manufacturers and depending upon how you calibrate the meter, and possibly also across the measurement range depending upon how the meter is programmed. You can calibrate the meter for either NaCl (most common, for aquariums) or 442 (more for hydroponics) - both have different factors that EC measurements are multiplied by, and hence give very different ppm readings.

For example, 700uS/cm conductivity could read 341 ppm TDS (if meter is calibrated with NaCl) or 481 ppm TDS (if calibrated with 442 solution). Almost makes you think that discussions of TDS are pretty meaningless given how much variation there could be if we were all to measure the same water with each of our meters!!!

Really it would be much better if we all swapped to EC meters and talked about conductivity, then we'd get rid of all this calibration / conversion uncertainty. But TDS and ppm is in our heads, so I guess we'll all continue to discuss and chase very precise TDS numbers that in reality have a massive amount of uncertainty.

TDS readings are really only any use in monitoring your own water conditions from one day to the next and getting a feel for how they are changing. If we want to compare measurements with each other, we really all need to switch to conductivity (EC), not TDS.

Apologies if the above is obvious to others, but it wasn't to me and I couldn't understand why my TDS readings were so different.


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Last edited by en7jos; 07-25-2020 at 02:58 AM. Reason: Typo
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post #15 of 15 (permalink) Old 08-04-2020, 05:21 PM Thread Starter
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Thanks for all the info, but I don’t want to get too into that lol, I just know a tds of around 180 has the best looking tanks, It doesn’t matter to me if we’re really reading tds or not, just want to get the same reading as some of the best looking tanks out there, and Im close now, I’m at 218 tds, and did some changes and my plants are growing like crazy and coloring up, I know it’s obviously not just tds that matters, but I mean having a tds of around 200 with balanced nutrients, cause it can be 200 tds with 150p nitrates plus whatever other stuff there is and that would be bad. But here’s a pic
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