GH – General Hardness

AKA: Permanent Hardness

GH is the measure of Calcium and Magnesium salts (to a small extent, other divalent cations). These are cations with a 2+ charge.  All fresh water sources contain calcium and magnesium in varying quantities. They form salts with anions (which have a negative charge). The most important of these are bicarbonate HCO3 and sulphate SO42-.

By definition,

1dGH = 10 mg/liter CaO

1dGH = 17.86 ppm in terms of CaCO3
The atomic weight of Mg is 24, that of Ca is 40, and that of CaCO3 is 100.  You are getting molar concentrations, pretending they are CaCO3, which neither is, and stating the answers in ppm…. (An assumption that leads to disasters if calculated incorrectly.)

More correctly represented as:

1 GH is 0.179 millimolar Ca++ or Mg ++.
This is derived from the definition: 10 mg CaO per litre of water.
The Molecular Weight. of CaO is 56, so 10/56 millimolar = 0.179

1 GH is 0.179 x 40 ppm Ca++  = 7.2 ppm Ca++
1 GH is 0.179 x 24 ppm Mg++  = 4.3 ppm Mg++

The effect on plants is direct – Calcium and Magnesium are required elements for growth.
The effect on fish – 90% of Calcium requirement is taken up by the gills…. then rest is from food.

Calcium can also limit the uptake of dangerous products like Zinc, Copper and Nitrite by blocking the pathway through the gills.

There are other electrolytes required by fish – Sodium (Na) and Potassium (K) are the main ones.  Due to solubility reasons, Na tends to be the most common element added to the aquarium.  All animal cells need a stable ratio of Na vs K.  Having a substantial K addition in quality Riftlake and GH Booster salts will assist this balance of Na:K .

 API Ca Test Kit  – new directions for use in Fresh Water:

  • Fill vial with 10ml of tank water (use a different vial than what is supplied).
  • Add 10 drops of Reagent 1
  • Shake Reagent 2 and add drop by drop until the test changes from Pink to Blue – each drop equals 10ppm Ca.

To work out the Mg content of an aquarium, test the water with an API GH Test kit and then with the API Ca test kit – add the results into the Excel spreadsheet (available for download below) to get an answer for Mg.  The desired ratio for Ca:Mg is 4:1 for all Planted, Fish and Shrimp tanks – total GH is dependent on the species in the tank.

GH Ca Mg Conversion Calculator