GH (general hardness)

GH measures the concentration of calcium and magnesium in water. In freshwater aquariums it is a key parameter for species compatibility.

Detailed explanation

The general hardness indicates how many calcium (Ca2+) and magnesium (Mg2+) ions are dissolved in the water. Soft water (low GH) is typical of Amazonian rivers. Hard water (high GH) is reminiscent of African ponds.

GH has an immediate practical role: it determines which fish you can successfully keep. Discus require GH 1-4. Malawi cichlids want GH 12-20. Putting a Discus in hard water or an African cichlid in soft water means chronic stress, frequent illnesses, impossible reproduction.

For aquatic plants, GH provides calcium and magnesium necessary for photosynthesis. Too low GH causes visible nutrient deficiencies: yellow leaves with green veins (magnesium deficiency).

In ponds, GH is very dependent on local mains water. Those who have hard water from the tap have an advantage for koi who prefer medium-hard water. Those who have soft water will have to integrate it.

In marine aquariums, GH is almost never measured because calcium and magnesium are controlled separately with specific tests.

Reference values by aquarium type

How it is measured

Drop test (JBL, Sera, API). The principle is similar to KH: drops are added until the colour changes. Each drop corresponds to 1 dGH. For the pond, test strips are a quick alternative.

What happens when it is too low

Thin shells in gastropods. Calcium and magnesium deficiencies in plants (yellow leaves). Fish that require hard water exhibit chronic stress.

What happens when it is too high

Impossible to keep pH low for Amazonian species. Acidophilic plants suffer. The water becomes visually clear but biologically incompatible for softwater species.

Related terms