What general hardness actually shows
General hardness (GH) mainly measures the concentration of calcium and magnesium ions in the water. Both minerals are essential for fish osmoregulation (their internal salt-water balance) and, for shrimp and snails, additionally crucial for building shell or exoskeleton.
GH and KH are often confused, but they measure different things: KH measures buffering capacity (bicarbonate), GH measures total calcium and magnesium content. Water can absolutely have high GH with low KH or vice versa — it's worth checking both separately.
Ideal range by stocking
| Stocking | GH range | Note |
|---|---|---|
| General community tank | 6 – 12 °dGH | Standard range |
| Discus, tetras, rasboras (soft water) | 2 – 6 °dGH | soft water desired |
| Malawi/Tanganyika cichlids, livebearers | 12 – 20 °dGH | hard water desired |
| Neocaridina shrimp | 6 – 8 °dGH | too-low GH makes molting harder |
| Caridina shrimp (e.g. bee shrimp) | 4 – 6 °dGH | noticeably softer water than Neocaridina |
Why GH fluctuates
The most common causes of change:
Evaporation: When water evaporates, minerals stay behind — GH creeps upward if you only top off with tap water instead of RO water.
Changing tap water: Your utility's GH can shift slightly by season.
Mineral-based decor: Limestone rocks or crushed coral continuously release minerals and slowly raise GH.
Remineralizing products: Deliberately dosed GH boosters for shrimp or hard-water fish raise the value on purpose.
What to do about deviations
If GH is too low: dedicated GH-boosting products (calcium/magnesium mineral salts) added directly to the water, dosed per the manufacturer's instructions. Especially important for shrimp, since too-low GH can cause molting problems.
If GH is too high: mixing in RO or distilled water during water changes lowers GH gradually. When topping off evaporated water, use only RO water instead of tap water to avoid a slow creep upward.