KH too low in the pond: why it threatens the entire balance, KH or GH to correct first, and the right method

In the water analyses we see, one parameter consistently tops the list of overlooked problems: KH. We think about algae, nitrites, and the pump, but rarely about it. Yet a KH that is too low is often the hidden cause of a pH that yo-yos, a struggling filter, and fish that are stressed for no apparent reason. Here is what KH really is, why it drops, and how to correct it in the right order.

KH, GH, pH: what does each do?

To convert: 1 °dH corresponds to approximately 17.8 mg/L expressed as calcium carbonate. Some tests display KH in °f (French degrees): 1 °dH is approximately 1.78 °f.

Why a low KH is dangerous

The pH becomes unstable

In a pond, pH naturally fluctuates throughout the day. During the day, algae and plants consume CO₂ for photosynthesis and the pH rises; at night, everything respires, CO₂ accumulates and the pH falls. Bicarbonates buffer these fluctuations. With a KH that is too low, the pH can vary by more than one unit between late afternoon and dawn, creating constant stress for fish.

The risk of a pH crash

When the buffer is depleted, the pH can collapse suddenly, for example after heavy rain, after a massive algae die-off, or simply due to the acidity produced by the filter. A pH that falls below 6 within a few hours can cause deaths, and a disease is often wrongly blamed.

The biological filter slows down

Nitrification, which converts ammonia into nitrites and then nitrates, consumes alkalinity: approximately 7 mg of alkalinity expressed as calcium carbonate for every milligram of ammonia nitrogen oxidized. A heavily stocked pond therefore “uses up” its KH, and bacteria slow down when pH and alkalinity decrease. The result: ammonia and nitrites accumulate.

Why KH decreases

• Rainwater, which is almost free of carbonates, dilutes the pond with every shower and every top-up.
• Filling with very soft water, low-mineral tap water or rainwater from a tank.
• A high biological load: many fish, lots of food, and therefore a lot of nitrification.
• The decomposition of organic matter, leaves and sludge, which produces acids.

KH or GH: which should be corrected first?

The answer is clear: KH first, if it is below 5 °dH. It protects the pH, and therefore the immediate survival of fish and filter efficiency. GH mainly affects long-term well-being, fish osmoregulation, and plant growth; a slightly low GH is rarely an emergency.

One important nuance: some calcium carbonate-based products, such as limestone chalk, raise both at the same time, gradually. KH+ products based on bicarbonate act faster, but only on KH. Therefore, always measure GH after correcting KH, and supplement with GH+ if necessary.

The right correction method

1. Measure with a drop test, which is more accurate than strips for KH.
2. Calculate the pond volume accurately. This is where all dosing errors come from.
3. Aim for +1 to +2 °dH per day at most. As a guideline, pure sodium bicarbonate raises KH by approximately 1 °dH at a rate of 30 g per m³; for a commercial product, follow the stated dosage.
4. Dissolve the product in a bucket of pond water and distribute it near a water inlet.
5. Measure again the next day, including pH, before the next addition.
6. Once the target has been reached, check weekly during the season and after every heavy rainfall.

Long-term stabilization

In soft-water regions or for ponds supplied by rainwater, a slow source of carbonates prevents repeated corrections. Coccolithic chalk, a natural calcium carbonate, dissolves gradually and maintains KH and GH over time. It is particularly suitable for large ponds and lakes.

You may also like

All methods compared: How to increase the KH of a pond or basin?

READ THE ARTICLE

Frequently asked questions

— What KH is suitable for a koi pond?

Around 6 to 8 °dH is a good target. Below 4 to 5 °dH, pH becomes unstable.

— Should KH or GH be corrected first?

KH, because it stabilizes pH. GH can then be corrected if necessary.

— Can KH be raised quickly?

It is better not to exceed 1 to 2 °dH per day, to avoid a sudden change in pH.

— Why does my KH keep dropping?

Most often due to rainwater, soft fill water, or a high fish load that consumes alkalinity.

In summary

• KH is the buffer that stabilizes pH.
• Aim for 6 to 8 °dH, never below 4 to 5 °dH.
• Correct KH before GH, in small steps.
• Measure with a drop test, and after every heavy rainfall.
• Calcareous chalk maintains KH and GH over time.

Last updated: September 28, 2026.