Duckweed Care Guide (Lemna minor)
Duckweed is the smallest floating plant most aquarists will ever keep, and one of the most familiar: a scatter of tiny, bright-green oval fronds, each only a few millimetres across, drifting over the water's surface and trailing a single fine root into the tank below. A floating plant is simply one that is not rooted in the substrate at all but sits at the surface, drawing its nutrients straight from the water. Hobbyists value duckweed for two things above all. It is one of the most effective nutrient-export plants you can add, quietly stripping the nitrogen and phosphate that would otherwise feed algae, and it makes an excellent live food for goldfish, cichlids, shrimp and other livestock. In the same breath, it is also one of the hardest plants to ever be fully rid of once it settles in, so it pays to go in with clear eyes. Our store lists it as Lesser Duckweed | Lemna minor.
1. Species Overview
Lemna minor, the lesser or common duckweed, belongs to the family Araceae – the arum family – where it now sits in the subfamily Lemnoideae. Older books place the duckweeds in their own family, Lemnaceae, and you will still see that name about; botanists have since folded them into the arums, so the two names describe the same group under a newer arrangement. Each plant is minute: a light-green frond (the small, flattened, leaf-like body) roughly one to eight millimetres long, usually joined in twos, threes or fours, with a single thread-like root one to two centimetres long hanging beneath each frond. That single root is a handy field mark, because the similar-looking greater duckweeds (Spirodela) carry several roots per frond and the even tinier Wolffia has none at all. Lemna minor is native across Africa, Asia, Europe and North America; it is not originally native to Australia but has become naturalised here, and we also have our own closely related native duckweed, Lemna disperma, which looks much the same to the eye. The store lists the plant simply as Lesser Duckweed. It almost never flowers in the aquarium, spreading instead by division, and in cold climates it survives winter as tiny starch-filled dormant buds called turions that sink to the bottom and resurface when the water warms.
2. Difficulty Level and Suitability
As a plant to grow, duckweed is about as easy as it gets, and here the sources are unanimous: Flowgrow rates it very easy, and every aquarium reference we checked agrees it is one of the hardiest plants in the trade. It needs no carbon dioxide injection, no special lighting and no substrate, it feeds itself straight from the water, and floating at the surface it is never shaded out. That makes it suitable for almost any freshwater set-up – a keen beginner's first tank, a low-tech planted aquarium, a shrimp or fish breeding tank where the cover shelters fry, and unheated coldwater or goldfish tanks, since it tolerates cool water well. The honesty this plant demands is not about keeping it alive but about keeping it in check. Duckweed grows explosively and is famously, genuinely, nearly impossible to remove once established: a single frond carried in on a net, on a new plant, or even on wet hands will seed a fresh colony, and it will happily blanket the entire surface if left alone. Weigh that up before you add it. Many experienced keepers prize it precisely for that appetite; others regard it as a weed they will never quite be rid of. Both are right, and which camp you land in depends entirely on how much surface-skimming you are willing to do.
3. Water Parameters
Steady, reasonably clean water matters far more to duckweed than hitting any exact figure, and the plant is remarkably tolerant, so treat the numbers below as comfortable targets rather than strict limits. The published ranges vary from source to source, which is itself a sign of how adaptable it is.
| Parameter | Target range | Notes |
|---|---|---|
| Temperature | 15–28 °C | Extremely hardy. Flowgrow lists a working range of 4–30 °C and aquariumbreeder 15–32 °C, with growth fastest in warm water. It is happy in a tropical tank and copes well with cool, unheated water – in cold conditions it simply goes dormant – which makes it a good choice for coldwater and goldfish tanks. |
| pH | 6.0–8.0 | Very adaptable. aquariumbreeder gives an optimum of 6.5–7.5 but survival from 5.0 to 9.0, and Flowgrow lists 5–9, so essentially any tap water suits it. A steady pH matters more than the exact figure. |
| GH (general hardness) | ~2–15 dGH (soft to hard) | GH is the calcium and magnesium in the water. Not fussy: aquariumbreeder lists roughly 2–15 dGH, and other sources record it thriving from soft to very hard water. Flowgrow does not quote a GH figure at all. |
| KH (carbonate hardness) | 1–12 dKH | KH is the water's buffering capacity. Undemanding: aquariumbreeder lists about 1–12 dKH, while Flowgrow records tolerance to around 21 dKH. Comfortable across the whole normal aquarium range. |
| Nutrients / substrate | Water-column feeder | Floating free, duckweed takes everything through its short roots and fronds directly from the water, so a nutrient substrate or root tabs do nothing for it. It is a hungry plant that strips nitrogen, phosphate and iron from the water column – which is exactly what makes it such an effective nutrient-control tool. |
The one thing to keep an eye on is that duckweed is a heavy feeder and needs some nutrients actually present to look its best. In a very lean tank the fronds can pale and shrink; a little nitrate and a complete trace mix, iron included, keeps them a healthy green. Conversely, if it is growing faster than you can crop it, deliberately lowering the nutrients in the water – through water changes and lighter feeding – is one of the few levers that will slow it down.
4. Lighting Requirements
On the same PAR framework our coral guides use (PAR is photosynthetically active radiation, a measure of usable light reaching the plant), low light for plants is roughly 30–50 PAR, medium 50–80 and high 80 or more. Duckweed is genuinely undemanding here, though the sources spread out: Flowgrow rates it low to high, aquariumbreeder leans medium to high for best growth, and others simply say it will grow under almost anything. In practice that disagreement barely matters, because a floating plant sits in the brightest, least-obstructed light in the tank. Even a modest aquarium light keeps it alive; stronger light simply drives it to multiply faster and denser. There is no need to acclimatise it to bright light the way you would a submersed plant. The catch runs the other way: a thick raft of duckweed is itself very effective shade and will cut the light reaching everything growing beneath it. That can be an asset over low-light plants such as anubias, ferns and crypts, but a heavy surface cover and a demanding carpet do not mix – thin the duckweed to control how much light passes through.
5. CO₂ and Fertilisation
Plants grow by photosynthesis, using light to combine carbon dioxide and dissolved nutrients into new tissue, so more available carbon usually means faster growth. Duckweed has the advantage every floating plant enjoys: with its fronds sitting on the surface, it can draw carbon dioxide straight from the air rather than relying on the little that is dissolved in the water. This is much of why floaters grow so fast unaided, and it means CO₂ injection is not required – Flowgrow lists it as entirely optional, and the plant will tolerate injected CO₂ without needing a scrap of it. What duckweed does respond to is nutrients, and this is where it earns its keep. Being a fast, hungry water-column feeder, it competes directly with algae for nitrogen and phosphate, and it is often described as one of the most efficient nitrate-stripping plants you can grow; a heavy mat can noticeably pull down nitrate in an over-fed tank. If you are running it deliberately as a nutrient sponge you may not need to dose it at all, but in a lean, well-scrubbed tank a general all-round liquid fertiliser carrying iron and trace elements keeps the fronds green rather than pale. Root tabs and nutrient substrates are wasted on a plant with no substrate roots, and liquid-carbon supplements serve no purpose here.
6. Planting and Placement
There is nothing to plant – duckweed is used the one way, floating free on the surface, so you simply tip it onto the water and leave it be. Being far smaller and tougher than large floaters such as frogbit or water lettuce, it is not troubled by a little water landing on its fronds and copes with more surface movement, though very turbulent, strongly agitated water will push it all to one end and pile it against the glass. The real placement question is containment. Left alone it spreads until it seals the whole surface, so many keepers corral it into one corner with a floating ring, a loop of airline tubing or a purpose-made surface divider, keeping an open patch for gas exchange, for viewing the fish and for any other floating plants you would rather keep separate. Be warned that duckweed does not respect these barriers for long – fronds slip past, ride the current and colonise the open water – so a divider slows it rather than stopping it. Keep it well away from filter intakes and overflow weirs, where it collects, clogs and gets drawn through into the filter.
7. Growth Rate and Propagation
Duckweed is among the fastest-growing flowering plants known, and its growth rate is really the whole story of the plant. Flowgrow and the aquarium references all rate it very fast; under good light and feeding a colony can double its numbers in a matter of days and blanket an open surface within a week or two. Propagation is not something you do so much as something you manage: each frond simply buds off new fronds from small pouches at its base, and these grow, detach and drift away as independent plants, so a pinch of duckweed becomes a mat entirely on its own. To propagate it deliberately you need only move a few fronds across to wherever you want more; far more often the job is the reverse – scooping out the surplus with a fine net or a jug as part of routine maintenance. In cooler tanks and ponds it can overwinter as those sunken turions and reappear when the water warms again, which is part of why it is so persistent.
8. Common Issues and Troubleshooting
Almost every problem with duckweed is a problem of too much rather than too little. The commonest is straightforward overgrowth: a complete surface blanket blocks light and slows the gas exchange the tank relies on, shading out and starving other plants and, if it seals the surface entirely, reducing the oxygen exchange your fish depend on. The fix is simple and endless – skim off the excess regularly with a fine net and never let it cover the whole surface. The problem that follows keepers around is removal. Getting duckweed out for good is notoriously difficult: it regrows from the smallest fragment, hides among other plants and hardscape, sticks to nets, hoses and hands, and rides in on new plants and fish bags, so full eradication usually means weeks of patient daily skimming, sometimes a blackout or a tank reset, and always quarantining and rinsing anything new before it goes in. Treat it as a plant far easier to add than to remove. Pale, yellowing or shrinking fronds are the opposite complaint and point to a nutrient shortage rather than disease – uncommon in a typical tank, but worth a complete fertiliser with iron if it appears. Do bear in mind that many fish treat duckweed as a salad: goldfish, koi, larger cichlids, silver dollars and turtles will graze or demolish it, which is a welcome free food in some tanks and the reason it never establishes in others. Finally, a word on responsibility. Duckweed is naturalised across much of Australia and a well-known nuisance in ponds, dams and slow waterways, where a small amount can carpet the water and rob it of light and oxygen. Never release duckweed, other aquarium plants, or tank water into drains or natural waterways; bin or compost any excess, and check the rules that apply in your state or region.
A Note on the Ranges Above
The parameters and other figures in this guide are a general guideline only. Every planted aquarium is different, and the ideal balance for duckweed depends on your lighting, your substrate, your other plants and whether you run CO₂. For help working out tank volume and the right dose of fertiliser, plant food and treatments for your system, have a look at our Aquarium Calculators.
Disclaimer
This guide is provided for general educational purposes only. It is not a substitute for professional advice, and we cannot guarantee outcomes. Individual plants vary, and the right conditions for your system depend on your lighting, your equipment and the other livestock and plants you keep. If you are unsure about anything, please get in touch with our team before proceeding.