General Hardness

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┬░GH - General Hardness

General, total or permanent hardness is a measure of the overall concentration of calcium, magnesium and other ions. It's measured in degrees, with one degree equal to about 17.9mg/l (17.86mg/l). The degree symbol is often replaced with a "d" (i.e. 6dGH or just 6dH). The harder the water, the higher the GH number.


  • Some countries measure GH in ppm (like the USA), others use the German unit dH.
  • Some countries use 'GCH' (General Carbonate Hardness) instead of 'GH'.
  • Some retailers (ADA) use TH abreviation (Total Hardness)
Levels of Hardness
Degrees Parts Per Million Description
0 - 4 dH 0 - 70 ppm very soft
4 - 8 dH 70 - 140 ppm soft
8 - 12 dH 140 - 210 ppm medium hard
12 - 18 dH 210 - 320 ppm fairly hard
18 - 30 dH 320 - 530 ppm hard
Higher > 530ppm Liquid rock (Lake Malawi and Los Angeles, CA)


Raising GH

If you live in an area with low GH water you may need to add chemicals to increase the GH level to a level suitable for your aquatic animals.

  • Calcium carbonate (CaCO3) is a typical and cheap material to do this. You can find it as Coral sand, coral pieces, sea shells, Cuttle-bone pieces, etc. It is also sold in gardening shops as 'Lime' usually in a powder form. It will however raise KH as well as GH. Which is probably a good thing if you live in a soft water area.
  • Magnesium sulphate (MgSO4) or more commoningly known as Epsom salt. This will raise GH without altering KH. Used in a ratio of 1:3 with CaSO4 (Calcium sulphate) (1 part MgSO4 to 3 parts CaSO4) to make your own GH mix for a freshwater aquarium.

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