Duckweeds in aquariums: the cursed plant that nevertheless purifies your water
F. MattierAll it takes is browsing a few enthusiast groups or forums to make a troubling observation: duckweed in an aquarium has a very bad reputation. Often called "green plague," it is the obsession of many aquarists who loudly ask how to get rid of duckweed once and for all. As soon as a small round leaf appears on the surface, panic sets in, and the hunt begins with a fish net.
Yet in natural aquariums, my philosophy is simple: nothing is dramatic as long as there is no excess. Trying to maintain a completely sterile tank, free of all spontaneous life, is an exhausting battle that goes against the laws of biology. Rather than rejecting this exceptionally vigorous plant outright, I invite you today to change your perspective. Let us learn together how it works, how to control its power, and how to make the most of all its benefits to provide absolutely pure water for your fish.
The truth about filters and the miracle of unlimited CO2
To understand why I defend this little plant so passionately, we must first dispel a persistent illusion about our equipment. Many beginners think their aquarium's mechanical filter completely cleans the water. This is false: no conventional biological or mechanical filter lowers nitrate levels. The filter merely transforms ammonia (extremely toxic) produced by fish into nitrites, then nitrates. The problem is that these nitrates eventually accumulate dangerously in the water.
How do you remove nitrates in a natural aquarium? There is only one answer: only plants can do it. In biology, it is always preferable for polluting mineral elements to be fixed and stabilized in the form of plant tissue rather than floating freely in the water column, where they eventually suffocate fish and feed unwanted algae.
This is where our much-maligned plant comes in. Why is duckweed such a formidable floating anti-nitrate plant? The secret lies in its position. Resting on the surface, it receives maximum exposure to the light from your aquarium lighting. But above all, it has completely unlimited access to the carbon dioxide (CO2) present in the air of your room! Conventional submerged plants are often limited in their growth because they lack dissolved CO2 underwater. Duckweed, on the other hand, knows no carbon dioxide shortage. Propelled by this rich air, it pumps pollutants and nitrates from your water at full speed.
Removing duckweeds means removing nitrates by hand
Of course, the philosophy of moderation means not letting nature overflow. You should not let the duckweeds multiply until they cover 100% of your tank's surface. If the layer becomes too thick, it will prevent gas exchange and plunge bottom plants into darkness that harms their development.
This is where the magic trick of low-tech aquariums comes in. Maintenance does not consist of eradicating the plant, but of managing it. Each week, during your tank maintenance, take a fish net and simply remove the excess handful of duckweeds to clear the surface.
Be aware of what you are doing at that precise moment: when you remove this handful of greenery from the water, you are literally physically extracting nitrates from your aquarium! You have turned invisible pollution (dissolved nitrogen) into solid matter (leaves). By throwing this excess into the compost or among your houseplants, you are carrying out the most effective decontamination possible. It is quite simply the only biological way to permanently remove nitrogen from your water's cycle.
The virtuous cycle: from waste to a gourmet meal
This prodigious plant machinery does not stop at simple purification. It is the starting point of an absolutely fascinating virtuous cycle. Nitrogen, which is initially toxic waste produced by your fish's metabolism (continuously released into the water through their gills and excrement), is captured by duckweed. The plant uses this waste as fertilizer to grow and biologically transforms it into rich, healthy plant proteins.
This process is an invaluable boon, especially if you keep heavy polluters. For example, goldfish and carp have an anatomical peculiarity: they have no stomach. Their digestive system is one long continuous tube that requires a massive supply of plant matter and fiber to function properly. Duckweed is simply their essential staple food in nature. By letting it grow in your tank (or in a separate tank), yesterday's harmful waste becomes today's vital resource.
If you would like to explore this fascinating subject of digestive well-being further, I strongly invite you to read our detailed article about duckweeds and goldfish.
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The alternative for cautious aquarists: giant duckweed
I know that despite all these biological explanations, some aquarists remain traumatized by past experiences and fear the propagation speed of common duckweed (Lemna minor).
If you are naturally cautious but still want to benefit from this purifying power, I have a perfect alternative to offer you: giant duckweed (Spirodela polyrhiza).
This relative has undeniable advantages. Its leaves are significantly larger, often beautifully purple underneath, and it develops a very elegant cluster of roots. Its greatest advantage lies in its cruising speed: it grows a little more slowly than the classic version, making it extremely easy to control. If you want to remove some, its broad leaves can be grabbed in the blink of an eye by hand or with a fish net. It offers exactly the same nitrate absorption capacity, but with an aesthetic that is highly appreciated in open tanks.
However, I should add a small clarification regarding the usefulness of duckweed in outdoor ponds: if you choose giant duckweed for your garden, be aware that it is sensitive to frost. Unlike common duckweed, which can withstand harsh winters, giant duckweed will disappear if you leave it outside when temperatures fall below zero. You will need to keep a culture indoors near a window so you can reintroduce it in spring!
👉 Choose beauty and peace of mind with our giant duckweed cultures.
Conclusion: make peace with living nature
Stop fighting nature unnecessarily. Aquarium keeping should not be a war of eradication, but a search for balance. By accepting the presence of this incredible plant machinery that is duckweed and moderating it with a simple weekly gesture, you transform a constraint into a major asset. You provide your fish with crystal-clear water, free of nitrates, and serve them a fresh, all-you-can-eat buffet of greenery. Nature does things very well; all you have to do is let it work for you.
Mattier
FAQ: managing floating plants
How can I prevent duckweeds from suffocating my aquarium?
The secret lies in regular maintenance, not total removal. During your weekly maintenance, or when changing the water, make a habit of skimming the surface with a fish net. The goal is to always leave about 30% of the water surface open and clear. This allows gas exchange (oxygen) to occur properly and the light from your fixture to reach the submerged plants at the bottom of the tank.
Do duckweeds survive winter in ponds?
The answer depends on the species chosen: common duckweed (Lemna minor) is very hardy and can overwinter at the bottom of the pond or withstand light frosts, then grow back vigorously in spring. Giant duckweed (Spirodela polyrhiza), on the other hand, is more fleshy and very sensitive to frost, and will die if water temperatures drop drastically. It is always wise to keep a small handful in a container of water indoors during the coldest months.
Do all fish eat duckweeds?
No, not all fish have a herbivorous diet! Goldfish, Koi carp, Ameca splendens, and some large vegetarian cichlids love them and can clear a covered surface in just a few days. Strict micropredators such as Betta splendens (the Siamese fighting fish), small rasboras, and tetras, however, will not touch them. For these species, duckweed will not serve as food, but it will perfectly fulfill its role as a biological anti-nitrate filter and reassuring cover.