Testing for General Hardness (Total Hardness) in Aquaponics

This is part of a series on Aquaponic Water Testing.


General Hardness is the sum of Calcium, Magnesium, Iron, Aluminum, Manganese, and any other divalent metal ion. In Aquaponic systems, generally only Calcium and Magnesium contribute meaningfully to General Hardness.

General Hardness is reported in 3 different ways; degrees (dGH), ppm, and grains per gallon (gpg). We are most concerned with ppm,  but dGH is a common measure, and you may see grains per gallon listed on a water quality report.

1dGH = 17.85ppm CaCO3
1gpg = 17.12ppm CaCO3

A common mistake I see online is people see that they have a GH reading of 100ppm, and think that means they have calcium of 70 and magnesium of 30.

GH (as ppm CaCO3) = CaCO3+ MgCO3 + FeCO3 + MnCO3

Because our water tests give us Calcium results as Ca2+, not CaCO3, we use a conversion factor to make the equation work.

GH (as ppm CaCO3) = Ca2+ * 2.5 + Mg2+ * 4.1 + Fe2+ * 1.79 + Mn2+ * 1.8

Someone with a GH reading of 100ppm could have a Calcium reading of 40, a Magnesium reading of 24, or some combination in the middle.

General Hardness Testing

Testing for General Hardness is done two ways: Titration, and Colorimetry.  Both methods work, although Colorimetry has problems with <10ppm values.

Titration based:

  1. API GH and KH Test Kit 1dKH resolution = 17.86ppm resolution
    $5.39 (@Amazon)
    Number of tests depends on your KH and GH levels, so cost per test varies
  2. Hach Digital Titrator + 8329 Sequential Hardness (10-4000)
    This sequential test provides Total, Calcium, and Magnesium Hardness
    $159@Hach + $84.95@Hach / 100 = $0.85eaHach Digital TItrator+Hardness (Ca and Total Sequential) HR Reagent Set
  3. Taylor Test K-1503. 2ppm or 10ppm resolution
    $60 with shipping as a kit.
  4. Taylor Test – based on K-1722. 10ppm resolution
    Order reagents:

    R-0854-C & R-0012-C
    and follow instructions here https://www.taylortechnologies.com/en/Image/GetDocument/725

Colorimeter based:

  1. Lamotte Smart3 + 4309-J (10-450ppm)
    $30 / 100 = $0.30ea

2 thoughts on “Testing for General Hardness (Total Hardness) in Aquaponics”

  1. Using your calculation method to derive Magnesium, and cheap aquarium testers I added magnesium to about 30ppm. However, I was seeing deficiency symptoms.
    Calcium 80ppm, GH 18°dGH = Mg 29.59 ppm.

    I purchased a Hanna HI 38033 Total Hardness test kit, which has a Total Hardness range up to 30 gpg, but was off the scale!

    Although it said 18° dGH, I suspect the aquarium tester was way off! This figures, since I had to add much more magnesium (from Epsom salt) than expected..

    Even after a 30% water change, GH is still above 30 gpg.. Do you think the Hanna tester is correct?

    1. Stop using cheap aquarium testers – especially for calcium. Stop by a pool supply store (or order online) and get the Taylor pool testing supplies I talk about.

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