GH & KH Explained
Two of the most confused numbers in the hobby. This guide breaks down what general hardness and carbonate hardness actually measure, why they matter for shell-building species, and how they relate to pH stability.
General Hardness (GH)
General hardness measures the concentration of dissolved calcium and magnesium ions in the water, usually expressed in degrees of general hardness (dGH) or parts per million. These minerals aren't a side note — they're the raw material snails and shrimp draw on to build and maintain their shells and exoskeletons, and they play a role in fish osmoregulation too.
Water described as "soft" has low GH — few dissolved minerals. "Hard" water has high GH. Shell-building species such as Malaysian Trumpet Snails, Ramshorns and Bladder Snails generally do best in moderate to hard water, where calcium is readily available. In soft water, shells can thin, pit or erode over time regardless of how well the animal is otherwise cared for.
Carbonate Hardness (KH)
Carbonate hardness measures carbonate and bicarbonate ions, expressed as dKH. Its job is different from GH — KH is essentially the water's buffering capacity, its ability to resist sudden pH swings.
Water with low KH has little buffering capacity. It can hold a stable pH for a while, but once that buffer is used up — often through the ongoing acid production of the nitrogen cycle — pH can crash suddenly rather than drifting gradually. Water with adequate KH resists that swing, keeping pH steadier day to day even as the tank does its normal biological work.
It's a common assumption that hard water automatically means high KH, but the two don't always move together — you can have water that's high in GH but low in KH, or the reverse. They're measuring different ions for different purposes, which is why a full water test covers both separately rather than inferring one from the other.
Why This Matters Day to Day
For most community setups, the practical takeaway is straightforward: know your tap water's GH and KH before you build a stocking plan around it, and match species to what your water naturally provides rather than fighting it constantly with chemical adjustment.
If you're keeping shell-building snails or shrimp and noticing shell erosion, pitting or a chalky appearance, GH is usually the first thing worth checking. If you're seeing unexplained pH swings between water changes, particularly in a heavily stocked or mature tank, KH is the more likely place to look.
Quick Reference
- — GH = dissolved calcium and magnesium — the mineral supply for shell and exoskeleton growth
- — KH = carbonate/bicarbonate buffering capacity — resistance to pH swings
- — The two are independent — test both, don't assume one from the other
- — Low GH → shell erosion risk in snails and shrimp
- — Low KH → unstable, potentially crashing pH between water changes
Important Context
Regional tap water varies considerably across the UK, so there's no single "correct" figure that applies everywhere. Test your own water before making any adjustment, and change hardness gradually — sudden shifts in GH or KH are more stressful to livestock than the original reading usually was.
Jack's Aquatics · Medway, Kent
UK Bred. Mineral Supported. Never Imported. — jacksaquatics.co.uk