How to get rid of duckweed in a pond?

Is your pond covered with duckweed, and does it return after every pass with the net? To control it sustainably, you need to remove the plants present and address the conditions that encourage their return. Our approach combines water analysis, aeration, and management of nutrients and sludge. Depending on the diagnosis, Nautex, OASE SeDox Max, and Aquipond Bactovase can complement this intervention.

Guide updated in October 2026.

Why does duckweed take over a pond?

Duckweed consists of small floating plants, not algae. They can arrive with new plants, birds, or an inflow of water. Their vegetative reproduction allows them to quickly colonize a calm surface. A dense mat deprives submerged plants of light and may contribute to a decrease in available oxygen.

A few duckweed plants have their place in a natural pond. The main goal is to prevent excessive coverage. When they keep returning, look for what is sustaining the proliferation: nutrient-rich water, decomposing leaves and vegetation, feeding, runoff, or sludge accumulation.

1. Analyze the water before choosing products

A photo shows the infestation; an analysis helps you understand how the pond functions. Also note the temperature, average depth, presence of fish, and sludge thickness.

A negative redox reading warrants investigation, but it is not by itself a treatment dosage. Compare measurements taken under similar conditions. For a large pond, a sediment analysis can complement the water analysis.

2. Remove the duckweed to take nutrients out of the pond

Use a fine-mesh net. Over a large area, a floating barrier can help gather the plants before collection. A suitable surface skimmer makes maintenance easier; check and empty its basket regularly.

Collecting the plants also removes the nutrients they contain. Avoid leaving the piles to decompose in the water or on a bank where rain could wash them back in. Inspect the harvest to return small animals to the pond. Do not push the duckweed into a nearby watercourse.

3. Restore suitable aeration for the water body

An aeration system sized for your pond supports biological processes and creates movement. Choose it according to the surface area, depth, and organic load. A small air pump for a decorative pond may not meet the needs of a large pond.

If fish are gasping for air at the surface, prioritize oxygenation and water monitoring. In a deep or heavily silted pond, have the installation approved to avoid sudden resuspension of sludge. Aeration supports restoration; it does not remove the duckweed for you.

4. Nautex: supporting mineralization and bottom treatment

Nautex is a coccolithic chalk made from calcium carbonate. It can help buffer an acidifying environment, flocculate particles, and support conditions favorable to the biological degradation of organic sediments. Its usefulness should be determined based on analyses and the condition of the bottom.

A distinction must be made between water mineralization, related in particular to hardness, and mineralization of organic matter, which refers to its biological transformation. Nautex supports the environment; it is not a herbicide that directly destroys duckweed.

The powder is prepared as a suspension in water, in the form of “Nautex milk,” and then spreads evenly. Follow the instructions and have the quantity confirmed according to the surface area and diagnosis. Prefer application when there is no frost or extreme heat. Nautex does not replace the aerator.

5. SeDox Max: immobilise the phosphate that sustains proliferation

When excess phosphates or their release from sediments has been identified, OASE SeDox Max can complement the programme. This mineral treatment is designed to bind phosphate in an insoluble form and reduce its availability in the environment.

This action targets the nutrients that fuel biomass production. It supports the fight against duckweed; it does not replace its removal and does not guarantee its disappearance. Continued inflow of nutrient-rich water can sustain proliferation despite treatment.

Prepare an estimated water volume based on the surface area and average depth. A 100 m² pond does not automatically have a volume of 100 m³. Have the dosage and application validated based on the instructions, analyses and characteristics of the body of water.

6. Bactovase: break down the organic fraction of the sludge

Aquipond Bactovase provides microorganisms to support the breakdown of organic deposits. It works on the bottom, where leaves, plant waste and other organic matter can accumulate.

Bactovase does not digest sand, clay or other mineral sediments. Its effectiveness depends on the nature of the sludge and the biological conditions. Appropriate aeration supports this step. Progress is monitored by measuring the sludge thickness in the same locations.

The solid form intended for ponds is spread over the surface to reach the deposits. Solid Bactovase and liquid Bactovase have different instructions for use: follow the instructions for the product selected. For large-area treatment, have the packaging and quantity confirmed before ordering.

How should Nautex, SeDox Max and Bactovase be used together?

The guiding principle is simple: diagnose, remove, oxygenate, then correct the problems identified. If the environment requires mineral support, Nautex can be a first step. If the phosphate load warrants treatment, SeDox Max targets this nutrient reserve. Bactovase then complements the work on organic deposits, with aeration maintained.

This program is tailored to your pond. The three products are not always necessary. Do not mix them in the same container, and do not assume they should be applied on the same day: determine the order, intervals, and doses based on the instructions and your test results.

Prevent duckweed from returning

• Remove leaves and plant accumulations before they decompose.
• Limit runoff from fertilized lawns and nutrient-rich areas.
• Adjust feeding to the fish population.
• Check water inflows and newly introduced plants.
• Keep the skimmer and aeration maintained.
• Track duckweed coverage, test results, and the condition of the bottom over the weeks.

Frequently asked questions about duckweed

Does a UV device eliminate duckweed?

Duckweed consists of floating plants. A UVC used to clarify green water does not replace mechanical removal.

Should I add ducks or grass carp?

We do not recommend introducing animals solely to resolve this invasion. They alter the balance of the body of water and may add organic matter. Have the population and context assessed before introducing any animals.

How long does it take to see results?

Removal produces an immediate visible change. Restoring the bottom requires longer-term monitoring. The rate of change depends on the environment, season, and nutrient inputs; we do not promise disappearance within a few days.

Can treatment be carried out in autumn?

Autumn is a good time to prepare the bottom and limit leaf accumulation. Adapt each intervention to the water temperature, weather, and product instructions.

Technical references: Nautex — TASO, SeDox Max — OASE, Bactovase — Aquipond and duckweed management — RHS.