Calcium, GH, and Magnesium Formula

[Mattlikesfish36]

I am trying to figure out the magnesium concentration in my aquarium and I am following the formula I have seen in other posts. (ppmGH - (2.5xCa ppm))/4.1= Mg ppm. I obtained my general hardness level with the API test kit (with the droppers and test tubes, not the strips) and the calcium level with the API test kit. I have attached a photo of my calculations, but don’t understand how I am getting a negative value. My GH is 125.3 ppm and calcium is 60 ppm. Has anyone experienced this before? @Seattle_Aquarist I am wondering if you have experienced this or if this makes any sense. I have seen your posts using this formula in the past. Thank you for any help you can offer! 

Edited April 22, 2022 by Mattlikesfish36

[modified lung]

That equation is right. That's a suspiciously high amount of calcium for the amount hardness.

[Mattlikesfish36]

@modified lungthanks for your reply! It has me stumped. I tested both values multiple times and keep getting the same results from my test kits 

[modified lung]

I just download the API calcium kit manual. It's definitely not made for freshwater which means low readings on the kit's scale are probably very inaccurate.

Try adding 10 drops of bottle #1 to a 20 mL sample of water instead of 5 mL. Then count each drop from bottle #2 as 5ppm Ca. See what you get.

Edited April 22, 2022 by modified lung

[Mattlikesfish36]

On 4/21/2022 at 10:04 PM, modified lung said:

I just download the API calcium kit manual. It's definitely not made for freshwater which means low readings on the kit's scale are probably very inaccurate.

Try adding 10 drops of bottle #1 to a 20 mL sample of water instead of 5 mL. Then count each drop from bottle #2 as 5ppm Ca. See what you get.

@modified lungthanks for that information. I will give that a shot and see what happens… just need to figure out a test tube that will hold 20ml. I read in the manual that it can test for levels down to 20ppm so I figured I would be safe to try it but it makes sense that it would be less accurate if it is designed for saltwater. 

@modified lungthe water quality report I found online for my region gives values of 26 ppm for calcium and 93 ppm for hardness. But both of these are off from my tests so I’m not sure which to trust 

[modified lung]

@Mattlikesfish36 it's not uncommon for hardness to increase somewhere between the water plant and your tap. Or if your region is like mine there might be a bunch of wells with very different levels and only the average might be reported. The average Ca here is 30 ppm and average Mg 19 ppm.

[Seattle_Aquarist]

Hi @Mattlikesfish36

I made a calculator using formulas in an Excel spread sheet and this is what it computed to be:


Obviously you do not have -6 ppm of magnesium in your tank, the API Calcium Test Kit is the likely issue.  I have used the Salifert Calcium Test Kit for about three (3) years and it works fine for freshwater.  Many of the websites state it can be used for "marine water, freshwater and garden pond water." It costs a little more than some kits but it works.  -Roy

Edited April 22, 2022 by Seattle_Aquarist

[Mattlikesfish36]

On 4/22/2022 at 1:03 AM, Seattle_Aquarist said:

Hi @Mattlikesfish36

I made a calculator using formulas in an Excel spread sheet and this is what it computed to be:


Obviously you do not have -6 ppm of magnesium in your tank, the API Calcium Test Kit is the likely issue.  I have used the Salifert Calcium Test Kit for about three (3) and it works fine for freshwater.  Many of the websites state it can be used for "marine water, freshwater and garden pond water." It costs a little more than some kits but it works.  -Roy

@Seattle_Aquaristthanks for the info! I’ll check that out and see if the results are any different from the API kit

[Mattlikesfish36]

On 4/21/2022 at 10:04 PM, modified lung said:

I just download the API calcium kit manual. It's definitely not made for freshwater which means low readings on the kit's scale are probably very inaccurate.

Try adding 10 drops of bottle #1 to a 20 mL sample of water instead of 5 mL. Then count each drop from bottle #2 as 5ppm Ca. See what you get.

I used your technique today and got 45ppm as a result. I ended up doing it with a 40ml sample and counting each drop as 2.5ppm as well, and I got 42.5 as a result. I guess 42.5ppm is much better than 60ppm. Thanks for your advice on adjusting the test for a more precise result! 

On 4/22/2022 at 8:11 AM, Mattlikesfish36 said:

@Seattle_Aquaristthanks for the info! I’ll check that out and see if the results are any different from the API kit

I got the salifert test today and I followed the directions exactly, but I cannot get the solution to change from blue to pink. I used the API kit with a modified amount of solution as noted above, and got 42.5 ppm. Seems much better than the 60ppm I initially thought I had. 

[modified lung]

On 4/23/2022 at 5:40 PM, Mattlikesfish36 said:

I used your technique today and got 45ppm as a result. I ended up doing it with a 40ml sample and counting each drop as 2.5ppm as well, and I got 42.5 as a result. I guess 42.5ppm is much better than 60ppm. Thanks for your advice on adjusting the test for a more precise result! 

I got the salifert test today and I followed the directions exactly, but I cannot get the solution to change from blue to pink. I used the API kit with a modified amount of solution as noted above, and got 42.5 ppm. Seems much better than the 60ppm I initially thought I had. 

Cool. 42 makes way more sense than 60. I was guessing it would be between 35 and 40 given your hardness and regional numbers. Let me know what the salifert test says if you get that working.

[Seattle_Aquarist]

On 4/23/2022 at 5:40 PM, Mattlikesfish36 said:

I got the salifert test today and I followed the directions exactly, but I cannot get the solution to change from blue to pink. I used the API kit with a modified amount of solution as noted above, and got 42.5 ppm. Seems much better than the 60ppm I initially thought I had. 

Hi @Mattlikesfish36

I've never had an issue with the Salifert test kit not reading.  I do like to hold it up against a white piece of paper to be able to see the color change.