From The Aquarium Wiki
°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'.
|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)|
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.