KH, or carbonate hardness, measures the carbonates and bicarbonates dissolved in your water. Its job is to buffer pH — it stops your aquarium’s pH from swinging or crashing. Low KH is the hidden cause behind most “my pH keeps dropping” problems.

Here is what KH is and what level you need.

What KH does

KH acts as a chemical buffer. Fish waste and decaying matter constantly produce acids that push pH down; KH neutralises those acids and holds pH steady. When KH is depleted, pH crashes suddenly — dangerous for fish.

KH is measured in degrees (dKH) or ppm. It is different from GH (general hardness), which measures calcium and magnesium.

Ideal KH levels

• Community tanks: 3–8 dKH is a stable range.
• Soft-water species (tetras, discus): lower KH is fine, but watch for pH stability.
• Hard-water species (mollies, African cichlids): 8+ dKH.
• Below 2 dKH: risky — pH can crash without warning.

How to adjust KH

To raise KH (and stabilise pH): add crushed coral or aragonite — see how to raise pH.

To lower KH: dilute with RO water or use peat filtration — see how to lower pH.

Test KH with an API GH/KH test kit.

Frequently asked questions

What is KH in an aquarium?

KH, or carbonate hardness, measures the carbonates and bicarbonates in your water. It buffers pH, preventing the swings and crashes that stress fish.

What should KH be in a fish tank?

3–8 dKH is a stable range for most community tanks. Hard-water species prefer 8 or higher, while a KH below 2 risks sudden pH crashes.

What is the difference between KH and GH?

KH (carbonate hardness) measures buffering capacity and controls pH stability. GH (general hardness) measures dissolved calcium and magnesium, which matters for fish and plant health.

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— Jaqline Smalling, FishForever