Duckweed in an aquarium: the complete guide to using it as a nitrate filter (without letting it take over)
On forums, duckweed has the worst reputation in the aquarium hobby: “green plague,” “impossible to eradicate,” “throw it out after the first leaf.” Yet it is one of the most effective plants for removing nitrates and phosphates from an aquarium. The problem isn't the plant. It's the lack of a method. In this guide, we give you what other articles gloss over: the exact mechanism, figures with orders of magnitude, the right coverage rate, the signals the plant gives you about your water quality, and a five-minute weekly maintenance protocol.
The essentials in 30 seconds
- Your filter does not lower nitrates: it produces them. Only water changes and plant growth remove them.
- Floating on the surface, duckweed has access to CO2 air and all the light: it grows faster than any submerged plant.
- Every handful removed with a net takes nitrogen and phosphorus permanently out of your tank.
- The right setting: 30 to 60% surface coverage, never 100%.
- Color, leaf size, and root length give you a free indication of your water's nutrient load.
What exactly is duckweed?
The name “duckweed” refers to a small family of floating flowering plants, the Lemnaceae, which includes five genera and around thirty-six species. They are not algae: they are true plants, with stomata, a small root (or none, depending on the genus) and, from time to time, tiny flowers that are almost invisible to the naked eye. The genus Wolffia also holds the record for the world's smallest flowering plant.
Its secret weapon is vegetative reproduction. Each “leaf” (technically a frond) produces new fronds along its edges, which detach and immediately start again. Under favorable conditions, the biomass can double in two to four days. That's what makes it formidable when left unchecked, and valuable when managed.
Why your filter doesn't lower nitrates
This is the misconception that costs beginners the most. A biological filter does not “clean” nitrogen: it transforms it. Fish release ammonia through their gills and waste, and uneaten food also releases it. Filter bacteria convert this highly toxic ammonia into nitrites (also toxic), then into nitrates, which are far less dangerous but accumulate week after week.
Except in very specific oxygen-free areas (deep substrate, dedicated denitrifying filters), nothing in a conventional aquarium makes these nitrates disappear. There are only two ways out: water changes, which dilute them, and plant growth, which transforms dissolved nitrogen into plant tissue. That is exactly where duckweed excels.
Why duckweed absorbs nutrients faster than your submerged plants
A plant that lives underwater is almost always limited by carbon: CO2 diffuses approximately ten thousand times more slowly in water than in air, and an aquarium without injection contains little of it. Submerged plants therefore grow slowly and absorb nitrogen slowly.
Duckweed plays by different rules. Its stomata are on the upper surface of the frond, in contact with the air: it has access to CO2 practically unlimited. It is also first in line for light, just beneath the light fixture. Its roots, meanwhile, are immersed in the water and draw nitrogen and phosphorus from it. The result: it combines the advantages of a terrestrial plant (carbon and light) with those of an aquatic plant (dissolved nutrients freely available).
A detail rarely mentioned: like most aquatic plants, it prefers ammonium to nitrates because assimilating it requires less energy. In a young or overcrowded tank, it therefore intercepts some of the nitrogen before it even becomes nitrite. This is an additional safety buffer, not a replacement for the filter.
How many nitrates does a handful actually remove?
You often read that duckweed “removes nitrates,” without ever seeing a figure. Let’s do the calculation using cautious values drawn from the literature on the cultivation of the duckweed family.
≈ 95%
of water in fresh duckweed
4–6 %
of nitrogen in dry matter (water-rich)
2–4 days
to double its biomass under good conditions
≈ 5 mg/L
fewer nitrates per 50 g removed from a 100 L tank
The calculation, step by step
50 g of drained lentils (a generous handful) → approximately 2.5 g of dry matter → approximately 0.12 g of nitrogen (at 5%) → approximately 0.55 g of nitrate ions equivalent (1 g of nitrogen corresponds to 4.4 g of nitrate). Diluted in 100 liters, this represents approximately 5 mg/L. This is equivalent to a 10–20% water change in a tank at 25–50 mg/L, without touching the water.
This figure is an estimate, not a promise: nitrogen levels vary according to the richness of the water, and some of the nitrogen absorbed comes from ammonium rather than nitrates. But it demonstrates one essential point: the effect does not come from the presence of duckweed; it comes from removing it regularly. A mat that is never harvested eventually dies, decomposes, and returns to the water what it had taken up.
The right coverage level: neither too much nor too little
A fully covered surface causes three specific problems. Bottom plants lack light and deteriorate. Gas exchange at the surface slows down, which is most noticeable toward the end of the night, when oxygen levels are at their lowest. And fish that breathe at the surface (bettas, gouramis, Corydoras that “come up” for air) can no longer reach it. By contrast, a few scattered fronds have almost no measurable effect.
Aim for 30 to 60% surface coverage. Above 60%, plan to harvest within the week. Above 80%, harvest that same day.
The 5-step maintenance method
1
Set off an area with a floating ring. A simple air-pump hose closed into a circle (or a feeding ring) is enough to keep the duckweed on one side of the tank. The rest of the surface remains clear for gas exchange and the fish. This is the one step that makes all the difference, and almost no one does it.
2
Harvest once a week, always at the same time. Before the water change, turn off the surface pump so the fronds are not dispersed. A fine-mesh aquarium net removes in thirty seconds whatever has spread beyond the designated area.
3
Put the harvest to good use. Compost, around houseplants, in the vegetable garden, or in a separate tank to feed goldfish. Never down the sink, and never in a pond or ditch: aquarium plants must not be released into the wild.
4
Check the filter intake and sponge. A few sucked-in fronds do no harm, but a buildup eventually reduces the flow. A pre-filter sponge on the intake solves the problem.
5
Measure once a month. A nitrate test and a phosphate test show whether harvesting is sufficient or whether you can space out your water changes. Without measurements, you are left guessing.
What your duckweed tells you about your water
Duckweed reacts within a few days to the amount of available nutrients. Properly interpreted, it is a free and permanent indicator, far more responsive than your background plants.
Bright green, doubling in less than a week
The water is nutrient-rich. It is doing its job, but it also signals a high nutrient input: generous feeding or a large population. Harvest more often.
Small, pale, or yellow fronds
Deficiency (nitrogen, iron, or potassium). Good news for your fish: the water is nutrient-poor. Harvest less; the plant has reached the limit of what the tank can provide.
Very long roots
The plant is “searching” for its nutrients farther away. A sign of poor, stable water. If your background plants are also struggling, overall fertilization is probably insufficient.
Brown patches or fronds that rot
Condensation droplets on the lid, surface flow that is too strong, or direct heat beneath the light. It is almost never a water-quality problem.
Duckweed and fish: who eats it and who ignores it
Rich in protein (often 20 to 35% of dry matter when the water is nutrient-rich), duckweed is a staple food for several herbivores. Goldfish and carp, which have no stomach and digest continuously, derive real digestive benefits from it.
| Species | Eats duckweed? | Practical consequence |
|---|---|---|
| Goldfish, fancy goldfish | Yes, eagerly | Grow it in a separate tank and distribute it |
| Metynnis (silver dollar fish) | Yes, it clears a surface in a few days | Impossible to keep in the same tank |
| Ameca splendens, mollies | Yes, regularly | Growth and consumption often balance out |
| Apple snails (Pomacea) | Yes | A useful supplement, not an eradication solution |
| Betta, tetras, neon tetras, rasboras | No | It filters and provides shelter, nothing more |
| Corydoras, Neocaridina shrimp | No (shrimp graze on the biofilm on the roots) | The roots become a food source for young shrimp |
Small duckweed, giant duckweed, or another floating plant?
If the growth rate of Lemna minor concerns you, other floating plants provide a comparable service with less monitoring. Here is how they compare in an aquarium.
| Plant | Size | Growth | Easy to remove | Good to know |
|---|---|---|---|---|
| Lemna minor | 2–4 mm | ●●●●● | Moderately | The most effective at nutrient removal per cm²; sticks to plants and equipment |
| Spirodela polyrhiza | 5–10 mm | ●●●●○ | Yes | Purple underside, clustered roots: the best compromise |
| Wolffia | 0.5–1.5 mm | ●●●●● | No | Passes through most fish nets; reserve for grow-out tanks |
| Salvinia | 1–2 cm | ●●●●○ | Yes | Fern with hairy leaves; does not tolerate condensation well |
| Limnobium laevigatum | 2–4 cm | ●●●○○ | Very easy | Long roots, ideal shelter for fry |
| Phyllanthus fluitans | 1–1.5 cm | ●●○○○ | Very easy | Turns red under strong light; more demanding, with a weaker purifying effect |
A note about greater duckweed. You sometimes read that it “dies in freezing temperatures”. In reality, in autumn, Spirodela polyrhiza produces starch-rich winter buds (turions) that sink to the bottom and rise again in spring. It is native to Belgium and France, by the way. In an aquarium, the question does not arise; in an outdoor pond, it disappears in winter but returns on its own.
You really don't want any? The removal method that works
Some tanks are not suitable for it: mirror-surface aquascaping, surface-dwelling fish, very low lid. In that case, aim for zero fronds, which requires consistency more than force.
- Turn off the filter and all surface agitation during the operation.
- Remove everything with a fine-mesh net, then run a paper towel along the panes at the waterline, over the hoses and across the exposed decorations.
- Inspect the filter strainer, sponge and chamber: fronds can survive there and reappear a few days later.
- Check the surface three times a week for four weeks. A single forgotten frond will become a mat again within a month. This is the step that makes the difference.
- Prevent reintroductions: rinse every new plant under running water, keep it in an observation bucket for a few days, and never pour transport-bag water into the tank.
The five most common mistakes
- Letting them cover 100% of the surface, thinking that “the more, the better”. It is harvesting that purifies the water, not the stock.
- Never harvesting: older fronds die and release what they had absorbed.
- Relying on them to compensate for overfeeding: they mask the problem for a few weeks, until they can no longer keep up.
- Placing them under a strong surface current: they clump together in one corner, become crumpled and turn brown.
- Throwing the excess into nature: this is how non-native species spread through ditches and ponds.
Useful equipment
Fine-mesh net 12 cm
For the weekly harvest
Sera nitrate test
Measure the real effect
Sera phosphate test
The other fuel for duckweed
🧪
All water tests
Strips, drops, smartphone analysis
And in a garden pond?
Outdoors, the logic is reversed: duckweed is no longer a tool to be managed, but often a mat that covers everything within two weeks and causes oxygen levels to plummet. We have devoted a complete guide to this case: duckweed-covered pond or lake: diagnosis and a plan to regain control.
In summary
Duckweed is neither a pest nor a miracle solution. It is an extremely efficient nutrient pump, but it only helps if it is harvested. Confined within a ring, removed every week, and observed as an indicator, it lowers your nitrates, limits algae by cutting off their food supply, feeds your herbivores, and teaches you to read your water. Left to itself, it eventually covers the entire surface and releases what it had absorbed. The whole difference comes down to five minutes a week.
Have a question about your tank? Our advisers at Aquiflor aquatic garden centre in Nodebais answer questions in-store and by email.
FAQ: Duckweed in aquariums
Is duckweed dangerous for fish?
No, the plant itself is harmless. The only risk comes from complete surface coverage, which reduces light and gas exchange. With 30 to 60% of the surface covered, there is no problem.
Where does the duckweed appearing in my aquarium come from?
Almost always from a newly purchased plant or water from a transport bag. A single frond is enough. This is why it is important to rinse and observe every new plant before introducing it.
Can duckweed replace water changes?
No. They remove nitrogen and phosphorus, but not the other substances that accumulate (dissolved organic compounds, hormones, excess minerals). They often allow you to space out or reduce water changes, but never eliminate them.
Why is my duckweed turning yellow or dying?
Yellowing most often indicates a deficiency (iron, nitrogen, potassium): your water is nutrient-poor, which is generally a good sign. Patches of browning are more likely caused by condensation from the lid, an overly strong surface current, or direct heat under the lighting.
Does duckweed prevent algae?
They limit them by consuming the same nutrients and partially shading the tank. They are not an absolute algae cure, but in a balanced tank, they are one of the most effective natural methods.
Is a lid necessary on an aquarium with duckweed?
It is not necessary, but if you have one, leave sufficient air space and prevent condensation droplets from falling back onto the fronds. Open tanks suit them best.
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