How can duckweed be removed safely for fish?

Is duckweed taking over your koi pond? Find out how to remove it without putting the fish at risk. The priority is to protect oxygenation, the biofilter and water quality.

Important: never pour shock chlorine into a fish pond or a natural swimming pond. Products must be compatible with the organisms present and their authorized uses.

Recognizing duckweed

Duckweed consists of small floating plants, not algae. It can form dense coverage in calm, nutrient-rich water. Distinguish it from filamentous algae and cyanobacteria, some blooms of which can be dangerous. A close-up photo helps with diagnosis.

Why fish may be in trouble

Dense coverage can alter light and surface exchanges. Above all, the decomposition of a large amount of biomass can consume dissolved oxygen. Observe the fish: abnormal breathing, surface gasping and mortality require urgent assessment.

Never use shock chlorine

Chlorine, bromine and certain copper-based products intended for swimming pools must not be poured into a fish pond. Gills, invertebrates and beneficial bacteria may be damaged. Even a product sold for ponds must be used only according to its authorized uses.

Test the water before treatment

Check dissolved oxygen, pH, temperature, nitrites and ammonia/ammonium. Depending on the context, also test orthophosphates and nitrogen. Clear water is not necessarily healthy. A single phosphate measurement does not reveal all nutrient inputs.

Mechanically remove the duckweed

Use a fine-mesh net, a floating boom or a harvesting method suited to the surface area. Avoid injuring the fish and stirring up the sediments aggressively. Remove the plants from the pond to prevent their return and remove some of the nutrients they contain.

Install suitable aeration

A properly sized aeration system can help maintain oxygen levels. A surface aerator and a bottom diffuser do not have the same effect. Aggressive mixing of a deep pond can disrupt its stratification. Have the equipment selected according to the volume, depth and fish.

Identify the source of the nutrients

Fertilizers, wastewater, leaves, animal waste, fish food and sediments can introduce nutrients. If these inputs persist, duckweed may return after cleaning. Limiting the source is often more sustainable than a succession of treatments.

Nautex, SeDox Max and Bactovase: what role do they play?

Nautex is coccolithic chalk for ponds, SeDox Max is intended for phosphorus management, and Bactovase for managing organic matter and silt. Their use must be justified by analysis and approved as safe for fish. Do not infer a dosage from a photograph. Extensive decomposition can reduce oxygen.

Preventing its return without harming living organisms

Maintain the filters, remove leaves, limit nutrient inputs, and monitor oxygen during hot weather. Harvest the duckweed as soon as it reappears. Document measurements and photographs over several seasons to distinguish a lasting result from a temporary improvement.

FAQ about fish and duckweed

Is duckweed toxic? Not systematically. However, dense coverage and its decomposition can contribute to oxygen depletion. Do fish eat duckweed? Some species consume it, but their introduction must comply with local regulations and the balance of the pond. Is an algaecide safe? Only if its use is authorized and its compatibility has been demonstrated.

Practical protocol: the first 48 hours

Upon arrival at the site: Photograph the surface and estimate the proportion covered by duckweed. Check whether the fish are breathing normally and whether the filter is working. In case of gasping, mortality, or an unusual odor, suspend treatments and have the dissolved oxygen checked immediately. An existing aeration device may be checked, but do not improvise deep mixing in a stratified pond.

The same day: If possible, measure dissolved oxygen, temperature, pH, nitrites, and ammonia/ammonium. Gradually remove the duckweed with a fine-mesh net or floating barrier. Let the fish move freely and avoid stirring up the silt. Remove the plants from the water and keep them away from the banks so they are not carried back into the pond.

The next day: Check the fish's behavior and the condition of the surface again. Clean the prefilters according to the instructions, without unnecessarily destroying beneficial bacteria. If the cover returns, investigate nutrient inputs and accumulation zones before choosing a treatment.

Which parameters should be measured and why?

Measure at comparable times to track trends. A single analysis does not replace a historical record. For a large pond, several sampling points may be necessary.

Why killing all the duckweed at once can be risky

A large quantity of plants that dies and then decomposes becomes an organic load. The microorganisms involved in decomposition consume oxygen, while the fish also need it. The risk may be aggravated by heat, a high biomass, and insufficient circulation. This is why mechanical harvesting beforehand is often preferable to a strategy of allowing the entire cover to die in the water.

A chemical or biological treatment must never be considered a simple cosmetic operation. Its suitability depends on the analysis, applicable regulations, the exact technical data sheet, and the presence of sensitive organisms. For a heavily covered pond, seek specialist advice before abruptly changing the ecosystem.

Choosing between a fine-mesh net, floating boom, and suction

Fine-mesh net: suitable for a small, accessible pond. Work slowly to avoid injuring the fish. Floating boom: useful for gathering wind-blown duckweed in a collection area. Mechanized harvesting: worth considering for a large body of water with suitable access. Pond vacuum: useful for certain deposits of silt and organic matter, but not sufficient on its own to remove a duckweed cover spanning several hundred square meters.

The equipment must be suited to the volume, depth, and banks. Check that there is no dangerous suction for small fish, amphibians, and invertebrates.

Our before-and-after observation method

To document an intervention, take a general photo from the same viewpoint, note the approximate area covered, and record the water parameters. Repeat the observations after one week, one month, and throughout the seasons. Record rainfall events, any fertilizer applications, and filter maintenance. This monitoring helps distinguish a temporary improvement from a lasting reduction in recurrence.

Foudebassin field experience: in 2026, our team observed the complete disappearance of duckweed at several sites after combined interventions. These results do not imply that a single product was responsible or that every pond will react in the same way. Dated photographic and analytical records will help better demonstrate the results obtained.

A diagnosis that protects your fish and your budget

Before recommending an intervention, gather five pieces of information: the pond’s surface area and depth, the species and approximate number of fish, the filtration system, available water tests, and the possible source of nutrients. A photo of the cover and technical room is also useful. This information helps avoid an unnecessary purchase or an incompatible product.

Also see our general guide to duckweed and our guide to recurrence, sludge, and phosphates. For personalized advice, contact the Foudebassin team with your measurements.

Products and equipment: choose according to the diagnosis

Solutions to discover at Foudebassin

The following products are not a universal treatment: check their compatibility and conditions of use before any application.

• Nautex – coccolithic chalk
• OASE SeDox Max
• Aquipond Bactovase 25 kg

For more information: our main guide to duckweed.

For tailored advice, contact Foudebassin with the volume, depth, photos, and available water analyses.