Pond KH and GH: how to measure water hardness and correct it

Water hardness is the value we most often forget to measure, yet it determines the stability of the entire pond. If KH is too low, pH can drop sharply overnight. Here is what KH and GH mean, how to measure them and how to correct them.

KH, GH and pH: what does what?

— KH, the pH buffer

KH measures dissolved carbonates and bicarbonates. They neutralize the acids produced by fish, bacteria and decomposition. When KH is depleted, pH is no longer buffered and can collapse, which is dangerous for fish and stops the filter bacteria from working.

— GH, the minerals

GH mainly measures calcium and magnesium. These minerals are useful for fish, plants and the overall health of the pond.

— pH, the result

pH largely depends on KH. A pH that varies significantly between morning and evening often indicates KH that is too low.

How to measure hardness

• Test strips provide several readings at once in one minute. Practical for regular monitoring.
• Drop tests are more accurate: count the drops needed for the color to change.

Take the water sample away from the filter outlet, a few centimeters below the surface.

Why KH decreases

• Rainwater, which is very soft, dilutes the pond.
• Biological activity: filtration and fish constantly consume KH.
• Soft fill water, depending on your region.

How to raise KH and GH

Use a product specifically designed for ponds, dosed according to the volume. Raise it gradually, in stages, and measure between additions. A combined product can raise GH and KH at the same time.

Also read

An unstable KH also encourages algae growth. For green water, read My pond water is green, what should I do?.

READ THE ARTICLE

Frequently asked questions

— My KH is at 0, what should I do in an emergency?

Measure the pH. If it is low or unstable, add a KH+ product at a moderate dose, check the oxygenation, and measure again the next day.

— Is a KH that is too high a problem?

Rarely. It may be accompanied by a high pH, for example in a new concrete pond. In this case, monitor the pH above all.

— What is the difference between °dKH and mg/l?

These are two units. 1 °dKH corresponds to approximately 17.8 mg/l of calcium carbonate.

In summary

• KH maintains the pH, while GH provides minerals.
• Guidelines: KH of at least 6 °dKH, GH around 8 to 12 °dGH, stable pH.
• Test monthly and after heavy rainfall.
• Correct it gradually with a suitable product.

Last updated: September 27, 2026.