Ideal water values for a koi pond: pH, KH, GH, nitrites... the reference table and how to read it

Two identical ponds, the same filter, the same koi, the same food. One stays pristine all year, while the other turns into green water, sick fish, and pH that yo-yos. The difference cannot be seen: it must be measured. A handful of parameters is enough to determine the actual state of your water, and one of them, KH, alone explains a large proportion of ponds that “fall out of balance” overnight. Here are the reference values we use and how to interpret them.

The reference table

What each parameter tells you

— KH, the invisible factor

KH measures dissolved carbonates and bicarbonates. It is the “buffer” of your water: it absorbs the acids constantly produced by respiration, nitrification, and decomposition. As long as it is sufficient, the pH remains stable. When it is depleted, the pH no longer has any safeguard and can collapse overnight, after heavy rain, or during a peak in biological activity. This is the most common explanation for ponds that “tip overnight”, with lethargic fish and a filter that stops working, because nitrifying bacteria themselves consume KH and slow down considerably in acidic water.

Ponds filled with rainwater, ponds in granite regions and ponds with very high fish loads are most at risk. Measure the KH at least once a month during the season.

— pH

More important than its exact value is its stability. A pH that rises from 7.5 in the morning to 9 in the afternoon in a very green pond stresses fish and makes ammonia much more toxic. A suitable KH and fewer algae reduce these fluctuations.

— GH

It measures calcium and magnesium. These minerals help regulate water exchange in fish, support their skeleton and scales, and benefit plants. A GH that is too low, typical of rainwater and soft water, makes fish more susceptible to stress and disease. It can be corrected gradually with suitable mineral salts.

— Ammonia and nitrites

They are toxic and should be at zero in a mature pond. Their presence indicates a filter that is too small, too new, over-cleaned or overwhelmed by the fish and food load.

— Nitrates and phosphates

They are only slightly toxic to fish at usual concentrations, but they feed algae. They are what cause green water and filamentous algae. Partial water changes, plants and sensible feeding keep them low.

Test strips or drop tests?

Test strips give you a one-minute snapshot of six parameters: perfect for regular monitoring. For accurate results, dip them for one second, shake off the excess, read them after the indicated time and in natural light, and keep the tube tightly closed. Drop tests are more precise, especially for KH and nitrites: use them to confirm a borderline value or monitor an adjustment.

Correct without sudden changes

1. Only one parameter at a time, starting with the most urgent: nitrites, then KH.
2. Gradually: increase the KH by no more than 1 to 2 °dH per day.
3. Retest 24 hours after each adjustment.
4. Address the cause: a KH that drops every month often indicates an excessive organic load or water that is too soft for topping up.

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Frequently asked questions

— What pH is suitable for a koi pond?

Between 7.5 and 8.5, with as little variation as possible between morning and afternoon. A stable pH of 7.2 is better than a pH that fluctuates between 7.5 and 9.

— Is a GH that is too low serious?

Not immediately, but a GH below 6 °dH deprives fish and plants of useful minerals and is often accompanied by KH that is too low. Correct it gradually with mineral salts suitable for the pond.

— How often should I test the water?

Once a week in spring and summer, once a month in autumn, and immediately if the fish behave abnormally or after servicing the filter.

— Are test strips reliable?

Yes for regular monitoring, provided you respect the reading time and keep the tube dry. For a borderline value, confirm with a drop test.

In summary

• Stable pH 7.5-8.5, KH 6-10 °dH, GH 8-16 °dH.
• Ammonia and nitrites at zero, nitrates and phosphates low.
• KH is the pH safeguard: monitor it as a priority.
• Correct one parameter at a time, gradually, and address the cause.

Last updated: September 28, 2026.