General hardness (GH)
GH is the parameter most closely tied to whether a species is a 'soft water' or 'hard water' animal, and it is quite separate from KH despite the two often moving together. It is reported in German degrees (°dGH) or in ppm as calcium carbonate; 1 °dGH = 17.85 ppm.
In one sentence
General hardness measures dissolved calcium and magnesium. It matters biologically — fish and invertebrates use those ions for osmoregulation, and shrimp and snails need calcium to build shell and exoskeleton.
How to read a measurement
Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.
| Band | Range | Verdict | What it means |
|---|---|---|---|
| Very soft | ≤ 3 | watch | Suits blackwater species. Too low for shrimp to moult reliably and for most livebearers. |
| Soft | 3–8 | acceptable | Suits most South American and South-east Asian species. |
| Moderately hard | 8–15 | target | Suits the widest range, including shrimp and snails. |
| Hard | 15–25 | acceptable | Suits livebearers, rift-lake cichlids and most snails. |
| Very hard | ≥ 25 | watch | Rift lake territory. Soft-water species do not do well here long term. |
Why invertebrates care more than fish
A shrimp builds a new exoskeleton at every moult and a snail grows shell continuously, and both draw calcium from the water. In very soft water shrimp fail to complete moults — the condition keepers call the 'white ring of death' — and snail shells pit and erode. A GH around 6–8 °dGH is a common working minimum for Neocaridina, and nerite snails do better higher still.
Changing it
- To lower GH: dilute with reverse-osmosis or distilled water. Nothing else works reliably.
- To raise GH: use a purpose-made remineralising salt, or add calcium and magnesium sources such as crushed coral or a GH booster.
- Changing GH also changes what happens to pH, because the mineral sources involved usually carry carbonate too.
How it is measured
- Titration test kit (drops until colour change)
- Test strips
- Local water supplier's published hardness figure
Typical aquarium range
- Typical
- 4–15 °dGH
Common misconceptions
| Often said | What is actually the case |
|---|---|
| GH and KH are the same thing. | GH is calcium and magnesium; KH is carbonate and bicarbonate. Water can be high in one and low in the other. Remineralising with the wrong product raises the wrong one. |