water
GH vs KH
Both are called 'hardness' and both are measured in German degrees. What is the difference and which one matters?
Short answer
GH measures calcium and magnesium and matters biologically — it is what shrimp and snails need and what defines a soft-water or hard-water species. KH measures carbonate and bicarbonate and matters for stability — it is what stops the pH moving.
Side by side
| Dimension | General hardness (GH) | Carbonate hardness (KH) | Favours |
|---|---|---|---|
| What it measures | Calcium and magnesium ions | Carbonate and bicarbonate | — |
| Biological role | Osmoregulation; shell and exoskeleton building | None directly | — |
| Chemical role | Little | Buffers against pH change | — |
| Decides species suitability | Yes — soft vs hard water species | Indirectly, via pH | General hardness (GH) |
| Falls over time between water changes | Slowly | Yes — consumed by nitrification | Neither |
| Raised by | GH booster, calcium and magnesium salts | Crushed coral, bicarbonate, aragonite | — |
| Lowered by | Dilution with RO water | Dilution with RO water; acids | — |
| Test if | Keeping shrimp, snails, or a species with a hardness requirement | Investigating pH instability or an old tank | — |
Which to choose
General hardness (GH)
Test GH when deciding what you can keep, and when shrimp or snails are involved.
Carbonate hardness (KH)
Test KH when the pH is moving, when a tank has gone a long time without water changes, or when running CO₂.
One caveat
They usually rise and fall together in tap water, which is why they get conflated — but a remineralising product that raises one may not raise the other, and RO water has neither.