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parameter°dKH

Carbonate hardness (KH)

KH is not a target in its own right so much as a stability parameter. It is consumed continuously by nitrification and by acids from decaying organics, so it falls over time between water changes, and a tank that runs out of KH becomes vulnerable to a sudden pH crash.

In one sentence

KH measures carbonate and bicarbonate, which neutralise acid. It is the tank's resistance to pH change — low KH means pH moves easily, high KH means it barely moves at all.

How to read a measurement

Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.

BandRangeVerdictWhat it means
Negligible≤ 1watchEssentially no buffering. pH can crash quickly. Deliberate in some blackwater and Caridina setups, dangerous by accident.
Low1–4acceptableSoft water. Watch pH between water changes.
Moderate4–8targetStable and forgiving.
High8–15acceptableVery stable; pH will sit high and resist lowering.
Very high≥ 15acceptableRift lake conditions.

The old-tank syndrome mechanism

In a tank with infrequent water changes, nitrification and organic acids gradually consume the carbonate buffer. Nothing appears to be wrong until the KH reaches near zero, at which point the pH falls sharply and quickly — often overnight. This is 'old tank syndrome', and the trigger is frequently the belated large water change that then swings everything back the other way.

Testing KH occasionally is the cheapest early warning available. A KH trending downward between water changes is the signal to change more water, not to add a buffering product on top of the problem.

What general hardness and carbonate hardness each measureTwo columns side by side. The left column, general hardness, counts calcium and magnesium ions, and matters for osmoregulation, shell and exoskeleton building, and egg development. The right column, carbonate hardness, counts carbonate and bicarbonate ions, and matters for buffering capacity — how far the pH will move when acid is added. A note beneath records that the two usually rise and fall together in tap water, which is why they are confused, and that they can be separated deliberately.GHGeneral hardnessCa²⁺ and Mg²⁺OsmoregulationShell and exoskeletonEgg developmentWhat the animal responds toKHCarbonate hardnessCO₃²⁻ and HCO₃⁻Buffering capacityHow far pH will moveCO₂ / pH relationWhat the water responds toThey usually rise and fall together in tap water, which is most of why they get confused.They can be separated: RO water remineralised with calcium chloride raises GH and not KH.A tank with low KH and a fish that needs high GH is a real and common combination.
GH and KH are quoted in the same unit family and measure different things. GH is what the animal's physiology responds to; KH is what stops the pH moving. Source: AquariumHQ water chemistry model

KH and CO₂ injection

In a planted tank running pressurised CO₂, KH and pH together are used to estimate dissolved CO₂. The relation assumes carbonate is the only significant buffer, which fails in tanks containing tannins, phosphate buffers or commercial pH adjusters — and in those tanks the estimate reads far too high.

How it is measured

  • Titration test kit
  • Test strips

Typical aquarium range

Typical
3–10 °dKH

Common misconceptions

Often saidWhat is actually the case
Raising KH raises pH permanently.Raising KH increases resistance to pH change and usually raises pH as a side effect, because the materials used carry carbonate. The two are related but not the same lever.