KH (carbonate hardness)

Carbonate / bicarbonate concentration in water — sometimes called alkalinity. Buffers pH against drops. Higher KH = more pH stability. 4-8 dKH is typical for community freshwater tanks.

KH (carbonate hardness) measures the water's buffering capacity — how strongly it resists pH changes. Water with high KH is harder to acidify; water with very low KH (<2 dKH) can experience sudden pH crashes ('old tank syndrome') as biological acids accumulate.

KH is consumed by acid-producing biological activity (the nitrogen cycle, CO₂ injection in planted tanks, driftwood tannins). When KH drops to near zero, pH becomes unstable.

Practical implication: low-KH tanks need more frequent water changes to replenish buffering capacity. High-KH tanks are more forgiving but harder to acidify if you want soft-water species.

Beginner-safe rule

Beginner-safe rule: if KH drops below 3 dKH, do a water change. If your tap KH is naturally very low and you want stability, add a small amount of crushed coral to the filter — slowly dissolves to raise KH.