Before treating algae, measure GH, KH, and pH: the mistake we see every week in water analyses

The scene repeats every week during the water tests we carry out: a green pond or one overrun with filamentous algae, two or three algae treatments already tried, and an analysis revealing a KH of 2 or 3. The most common mistake is not choosing the wrong product, but treating algae without checking whether the water can withstand the treatment. Three values are enough to avoid most failures and accidents: KH, GH, and pH.

What happens when you kill algae

A mass of algae is living organic matter. When a treatment kills them, this biomass sinks and decomposes. Three consequences occur at the same time:

1. Oxygen levels fall, because the bacteria that break down dead algae consume a lot of it, while the algae themselves no longer produce any.
2. CO₂ and organic acids increase, causing the pH to drop.
3. Nutrients are released back into the water: phosphates and nitrogen become available again for the next wave of algae.

In well-buffered water, KH absorbs the acid shock; in water with KH 2, the pH can drop by more than one unit overnight. This is how fish die a few days after treatment, without the product itself being to blame.

The 3 values and what they tell you

KH, the treatment safeguard

This is the decisive value. A low KH means there is no reserve left to neutralize the acidity released by decomposition. It also explains some recurrences: filter bacteria work poorly in water without a buffer, waste accumulates, and algae benefit from the nutrients.

GH, an indicator of the source water

A very low GH almost always occurs with a low KH: a pond filled with rainwater or soft water. Calcium and magnesium are useful to fish and plants, which compete with the algae. Water that is too poor in minerals hinders precisely the plants you want to gain the upper hand.

pH, an indicator of algae activity

Measure it at dawn and in the late afternoon. A pH that rises sharply during the day shows that the algae are photosynthesizing intensely and that the buffer is insufficient. It is also a risk indicator: a very high pH makes ammonium more toxic.

The right order of intervention

1. Test: KH, GH, pH in the morning and evening, then phosphates and nitrates.
2. Correct the KH gradually, by one to two degrees per day, until it reaches 6 to 8 °dH.
3. Remineralize if the GH is low.
4. Remove mechanically as much filamentous algae as possible before any treatment, to limit the biomass that will decompose.
5. Treat, preferably in cool weather, while aerating well over the following days.
6. Treat the causes: phosphates, overfeeding, excessive sunlight, lack of plants.

Why this error is so common

Because algae are visible and KH is not. Because many test strips do not measure KH accurately. And because ponds filled with rainwater, very popular for ecological reasons, start with water that is almost devoid of buffering capacity. The right reflex can be summed up in one sentence: no algae treatment without a KH measurement taken less than a week ago.

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Frequently asked questions

— Why measure KH before treating algae?

Because the decomposition of dead algae acidifies the water. Without sufficient KH, the pH can drop sharply and endanger the fish.

— What is the minimum KH before an algae treatment?

Aim for at least 6 °dH. Below 5 °dH, correct it first.

— Should the algae be removed before treatment?

Yes, whenever possible: the less biomass that decomposes, the less oxygen and pH drop.

— Why do algae return after treatment?

Because the nutrients remain in the water and the cause is not treated: phosphates, food, light, lack of plants.

In summary

• Killing algae consumes oxygen and acidifies the water.
• Without sufficient KH, the pH may collapse after treatment.
• Measure KH, GH and pH before any treatment.
• Correct the KH, then the GH, remove the algae, then treat.
• Always treat the causes, otherwise the algae will return.

Last updated: September 28, 2026.