Testing for Magnesium in Aquaponics

This article on Magnesium Testing is part of a series on Aquaponic Water Testing.


Magnesium is important for photosynthesis in plants. It forms the center of the chlorophyll molecule. If deficient, plants will begin breaking down chlorophyll in older leaves, leading to the symptom of Interveinal Chlorosis in those older leaves.

Beware, Zinc deficiency produces the same symptoms. Ensure your magnesium levels are correct before considering Zinc. If you have interveinal chlorosis in older leaves and your Mg2+ levels are fine, take the time to test for zinc.

Hoagland suggests a Magnesium level of 48 ppm. The ratio of Magnesium to Calcium is important as well.

Measuring Magnesium

Your magnesium level can be measured, or calculated. Calculation is the traditional method. The Red Sea kit is an example of a direct titration, while the YSI Magnicol is a colorimetric test.

  1. Red Sea Magnesium Pro (Resolution 20ppm)
    $30.36 / 100 = $0.30ea (@Amazon)
    Red Sea Magnesium Pro Test Kit
  2. YSI Magnesium Magnicol (0-100; MDL 2; ±1)
    $38.40 / 50 = $0.77 OR $62.64 / 250 = $0.25

Calculating Magnesium.

Total Hardness = Calcium Hardness + Magnesium Hardness + divalent metal hardness.

We can safely ignore (and some tests specifically mask out) metal hardness in our tests.  That means if we test for Total Hardness and Calcium (or Calcium Hardness), we can calculate Magnesium Hardness. When calculating, your results will only be as accurate as your source measurements. Use a higher resolution method for the Calcium test, such as with the Hach Digital Titrator, or pool chemicals in order to get accurate results here.

Most calcium tests give the results in the form of Ca2+. In order to convert that to Calcium Hardness, we multiple it by 2.5.

Total Hardness = (Ca2+* 2.5) + Magnesium Hardness

Magnesium Hardness = Total Hardness – (Ca2+* 2.5)

Once we do our subtraction, we can convert Magnesium Hardness to Mg2+ by dividing by 4.1

Mg2+ = Magnesium Hardness / 4.1


Know of a different way to test for Magnesium in an Aquaponic system? Let me know in the comments and I’ll update the post.

One thought on “Testing for Magnesium in Aquaponics”

  1. I have a question about the desired ratio of calcium to magnesium. Using the above calculations, let’s say I have a GH of 16 gpg, and Ca2+ of 60 ppm (therefore 30.22 Mg2+).

    Would these levels be desirable in aquaponics? I can’t seem to find any information about it…

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