KH, or carbonate hardness, measures carbonates and bicarbonates that neutralise acids in aquarium water. Higher KH usually means pH is more resistant to sudden change; very low KH means normal biological acids can move pH more easily. KH is commonly measured in dKH or ppm. The right level depends on the species and system, especially because some soft-water animals prefer low KH while hard-water species prefer more buffering.
If pH is the number everyone watches, KH is one of the reasons that number behaves the way it does. It explains why one aquarium barely changes after an acid is added while another can experience a sudden pH drop.
What KH measures
KH reflects dissolved carbonate and bicarbonate buffering. These compounds react with acids and slow changes in pH.
That is why KH is often described as the aquarium's buffering capacity.
Why low KH can be risky
In a low-KH aquarium, acids from nitrification, organic breakdown and other processes have less buffer to consume before pH starts falling.
A deliberately soft-water aquarium can run low KH successfully, but it needs closer observation and a husbandry plan designed for that chemistry.
Why high KH makes pH difficult to lower
If KH is high, added acid is neutralised until enough buffering has been consumed. This can make pH-lowering chemicals appear ineffective or cause a temporary change followed by rebound.
Reducing the underlying mineral content is more predictable when genuinely softer water is required.
KH and cycling
Nitrification produces acids and consumes alkalinity. In very soft water, a long fishless cycle can therefore reduce KH and pH enough to slow bacterial activity.
If a cycle appears stalled, checking KH and pH can reveal a problem that an ammonia test alone will miss.
How to change KH
Crushed coral and appropriate alkaline buffers can raise KH. Mixing with RO or deionised water can lower high KH when softer water is required.
Always change KH gradually because changing buffering also changes how pH behaves.
Related aquarium tools and guides
Frequently asked questions
Is KH the same as GH?
No. KH is carbonate buffering; GH mainly measures calcium and magnesium hardness.
Does high KH always mean high pH?
They are related, but not identical. High KH tends to resist downward pH change and often accompanies alkaline water.
Can cycling lower KH?
Yes. Nitrification produces acids and consumes alkalinity, which can matter in very soft water.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.