Water Hardness (GH & KH)
The dissolved-mineral content of your water — GH affects fish, KH buffers pH, and together they explain how stable your tank really is.
Also known as: General hardness, Carbonate hardness, Buffering
Water hardness describes the dissolved minerals in aquarium water. General hardness (GH) measures calcium and magnesium and affects fish and invertebrate health; carbonate hardness (KH) measures buffering capacity and controls how resistant your pH is to change. Both are as important as pH, and often more useful to understand.
What it is
"Hardness" refers to dissolved minerals in the water, and in the aquarium it splits into two related measures that beginners often confuse.
General hardness (GH) measures the concentration of calcium and magnesium ions. It matters to fish and invertebrates directly: soft-water species from blackwater rivers prefer low GH, while livebearers, African cichlids and snails need higher GH for health and, for invertebrates, for building shells and moulting. GH is usually expressed in degrees (dGH) or ppm.
Carbonate hardness (KH), also called alkalinity or buffering capacity, measures carbonates and bicarbonates. Its job is to buffer pH — to resist changes in acidity. This is the crucial link between hardness and pH: a tank with adequate KH holds a stable pH, while a tank with very low KH can suffer sudden, dangerous pH crashes as acids accumulate between water changes.
Why it matters. Understanding hardness explains a lot that pH alone cannot. If your pH keeps dropping between water changes, low KH is usually why; if your snails' shells erode or shrimp fail to moult, low GH is often the cause. Matching fish to your source water's hardness — rather than only its pH — leads to healthier, more stable tanks.
Adjusting it. As with pH, work with your water where possible. Crushed coral or aragonite raises GH and KH and stabilises pH; reverse-osmosis water, peat or driftwood softens water for soft-water species. Test your tap water's GH and KH once, and let those numbers guide both your stocking and your maintenance schedule.
Worked example
A keeper's pH keeps crashing from 7.2 to 6.2 a few days after each water change, stressing the fish. Testing reveals very low KH — almost no buffering. Adding a small amount of crushed coral to the filter raises KH, and the pH now holds steady near 7.4 between changes. The problem was never pH itself but the hardness (KH) that controls it — the reason understanding GH and KH is so valuable.
Related entries
Sources & further reading
- Understanding Water Hardness in Aquariums — The Spruce Pets (article)
- Hard Water — Encyclopaedia Britannica (article)