General hardness (GH)
The total calcium + magnesium concentration. The number that decides whether livebearers thrive or shrimp moult cleanly.
What it is
General hardness measures the total concentration of divalent cations in the water — practically, calcium (Ca²⁺) and magnesium (Mg²⁺). Measured in degrees German hardness (°dGH); 1 °dGH ≈ 17.86 ppm CaCO₃ ≈ 10 ppm Ca + 6 ppm Mg (roughly, the 4:1 Ca:Mg ratio of tap water).
Why it matters
GH is the parameter livestock care about most after temperature. Hard-water species (livebearers, African cichlids, most snails and shrimp that build shells or exoskeletons) need GH above 8 °dGH for healthy moults and shells. Soft-water species (most South American tetras, discus, wild Apistogramma, cherry shrimp at the low end) breed reliably only below 6 °dGH.
GH and KH are independent: tap water from a chalk aquifer is high in both, but a RO + Equilibrium remineralised tank can have any combination. The hardness converter translates between °dGH, ppm, and the Ca/Mg breakdown your test kit may report.
How to test
Drop tests, counting drops to colour change. Test water at source (tap or RO mix) and in the tank — substrate and rocks can shift GH over time. Once a month is usually enough in a stable tank.
What high and low look like
Below 3 °dGH in a tank with calcium-hungry inhabitants: shrimp moult-fail, snail shells pit and erode, fish develop fin curl from the underlying calcium deficiency. Above 18 °dGH in a soft-water tank: Apistogramma fail to spawn, soft-water tetras stop colouring up.
How to fix
Too low — dose a remineraliser (Salty Shrimp GH/KH+, Seachem Equilibrium, JBL Aquadur). Too high — dilute with RO water; the simplest path is to switch the source water rather than fight the GH downstream. A 50:50 RO/tap mix typically halves your starting GH.
