Duckweed In Aquarium: Benefit Or Nuisance?

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If you’ve had problems with freshwater algae or maintaining water quality, you may have been recommended a type of floating plant called duckweed. Aquarium duckweed is generally a small plant that sits on top of the surface of many lakes and ponds in the wild.

It has been added to the aquarium hobby for nutrient control as well as for aesthetics. While these plants are very efficient at taking up nutrients, their fast growth rate has often led to depleted parameters.

Keep reading to find out everything you need to know about aquatic duckweed care and if this plant will be more of a benefit or a nuisance for your freshwater aquarium!

Name

Duckweed belongs to the Araceae family and is a flowering plant even though many species have microscopic flowers; some species within this family are the smallest flowering plants currently known to science.

Most aquarium duckweed species within the hobby belong to the Lemna genus. Because of its ability to quickly uptake nutrients, duckweed has been used as a controlled way to reduce excess nutrients within some waterways; this duckweed is then often harvested and used for livestock feed and fertilizer.

Currently, scientists are also working on extracting certain elements, namely proteins, from modified duckweed for more commercial biomedical advances.

Natural habitat

Duckweed can be found on almost every continent in slow-moving lakes, rivers, and pond systems. It is usually seen growing in large mats on the surface of the water, which can first appear to be algae if you don’t know what you’re looking at. It is considered a nonnative plant in Australia and South America.

While duckweed grows best in tropical settings with full light, it has been known to occur in a water temperature range between 42-92° F (5.6-33.3° C) with lower light settings. It does best in water with a more neutral pH around 7.0 but can adapt to more acidic or basic conditions.

In the wild, duckweed serves as protection for many fish, insects, amphibians, and other aquatic species. The large, dense mats also provide a sanctuary for many larvae and fry to mature. At the same time, the duckweed provides a rich food source for many of those same species, including birds as well.

Identification

macro shot of duckweed

The different species of duckweed can be difficult to tell apart from each other because many of the defining features are microscopic. In the aquarium hobby, you’re most likely to come across common duckweed (Lemna minor).

Common duckweed can have up to four leaves per individual plant. It is common to see one 0.4-0.8 inch (1-2 cm) root hanging from the center of the plant. Each small green leaf is usually rounded or in an oval shape and only a few millimeters long and wide. These plants do produce flowers on rare occasions but are too small to see most times.

In ideal temperature and water conditions, duckweed populations can double in size overnight. These aquatic plants continue to grow until eventually splitting into separate plants. The roots have been optimized for sticking onto surfaces, including the legs and feet of birds, in order to expand populations across aquatic systems.

Is duckweed good for your aquarium?

So far it seems like duckweed could be nothing but good for aquarium, however, many hobbyists despise duckweed and don’t even want it within the same room as their tank. Why is this?

There are many advantages to keeping duckweed, like nutrient control, protection, shade, and aesthetics. But for such a plant that can do such good, there seem to be many downsides to actually keeping duckweed in your fish tank, like nutrient uptake, equipment clog, and shade.

Advantages to duckweed

Here are some reasons why you might want to introduce this small plant into your freshwater aquarium.

Nutrient control

Duckweed growth rate is one of the fastest in the hobby. However, in order for these plants to grow, they need a lot of important nutrients like nitrates and phosphates, which are the main nutrients that often facilitate algae growth as well.

Because of this, many aquarists add duckweed to their fish tank as a ‘nitrate sink’ to help outcompete more nuisance algae species; the duckweed grows so quickly and takes up so many nutrients from the water that algae does not get the chance to establish itself. Duckweed is also recommended for hobbyists who might already be struggling with an algae problem.

However, duckweed doesn’t need to be used only when algae is present. If you have a water source with unfavorable parameters, this aquatic plant can also help keep nutrients under control and create a more stable system overall.

Protection

Like in the wild, duckweed can offer fish, invertebrates, and amphibians protection in the aquarium. Some fish will naturally stay towards areas of the tank with more coverage. This may be especially beneficial if the rest of the aquascape is minimal and there are not many other places to hide.

Duckweed can also be a good addition to breeding or spawning tanks as fish fry and other larvae will feel extra safe under the protection of dense coverage.

Shade

While duckweed will offer protection to your fish and invertebrates, it will also help shade lower plants from intense lighting. If you’re having difficulty with getting the right light spread for your aquatic plants, then these floating plants can be maintained to only cover certain areas of the fish tank, creating pockets of shade for less-demanding species.

Shade will also further help deter algae from growing in the aquarium by limiting excess light.

Aesthetic

In general, floating plants help bring another dynamic to the freshwater aquarium. These lush and easy plants can be added to most aquarium systems to bring a pop of color or excitement to the top of a tank.

Duckweed will also move with the flow of the water surface agitation, which can really be mesmerizing to watch after a while!

Disadvantages to duckweed

Here are a few reasons why hobbyists keep this plant as far away from their fish tanks as possible!

Nutrient uptake

While duckweed can be an incredible nutrient export, it doesn’t stop uptaking nutrients once the excess ones have been removed.

As duckweed uptakes more nutrients, it can reproduce even quicker which eventually leads to more duckweed plants taking up even more nutrients! This cycle needs to be closely regulated if you want to successfully grow other plants in the aquarium as duckweed can quickly deplete the tank of essential nutrients. In order to keep nutrients in check, it’s best to manually remove some duckweed every time it begins to grow a little too fast.

If you do not monitor the growth rate of your duckweed population, it has the ability to deplete the fish tank of all nutrients and then cause die off, which can cause an ammonia spike and a subsequent water cycle to begin.

It is also important to note that if duckweed is allowed to cover the entirety of the water surface, it can cut off the gas exchange with the rest of the inhabitants in the tank.

Equipment clog

Because duckweed can get so out of hand so fast, it often becomes a pain for hobbyists to keep out of their aquarium equipment. Duckweed likes to get caught up in filter intakes and powerheads and can be difficult to get out. This additional maintenance is something to consider if you plan on adding duckweed to your aquarium.

In general, it can be very difficult to remove a duckweed colony from the tank at any given point; this aquarium plant is one of those that seems to come back no matter how many times you try to get rid of it.

Shade

Just as duckweed can be used to shade portions of the fish tank for low light-demanding species, it can unintentionally shade areas that might need high light.

Plants need light to undergo photosynthesis and to grow. Because duckweed is closer to the source of the light than most other aquatic plants, it has optimal conditions to outcompete most other species. This means that other high light-demanding species can easily be blocked from receiving the amount of light they need to thrive.

Is duckweed harmful to fish?

It is very unlikely for duckweed to become harmful to fish or other invertebrates as long as it is not left to grow out of control.

As mentioned before, duckweed can quickly cover the top of aquarium and pond systems, shading large portions of the ecosystem. If other plants are growing in that area and are unable to receive sufficient light and nutrients, then those plants can die.

This can lead to a mass die-off, which can then cause anaerobic conditions due to decomposition and a decrease of overall oxygen production within the water column. At this point, many organisms would be unable to sustain such oxygen-deprived conditions.

How fast does duckweed grow in the aquarium?

Duckweed has been known to double in population overnight. This is not an exaggeration, and you should be prepared to deal with a constantly growing plant.

How do you control duckweed in the aquarium?

Many hobbyists avoid duckweed in their aquariums and ponds at all costs. While this flowering plant can help outcompete algae and keep water conditions optimal, the hassle of upkeep usually outweighs the benefits.

Manual removal and surface agitation

The best way to control duckweed is to manually remove some every once and a while. In most cases, you will need to do this every week or every other week. In aquariums, you may use your hand or a net; in a pond, it would be best to use a skimmer on a windy day so that all the plants are pushed to one side.

Another way to control duckweed is to increase the amount of water surface agitation. Most floating plants, including duckweed, do not grow well when they get wet on the top of their leaves as this interferes with gas exchange.

However, with this method, the plants may begin to rot and will need to be removed immediately. In pond systems, it’s best to use an aerator or other fountain mechanism.

Goldfish, koi, and other fish

duckweed on top of aquarium

If you have space in your aquarium or pond and have the system set up properly, it may be worthwhile to introduce aquarium fish species that use duckweed as a food source. Many pond fish species, like goldfish, carp, koi, and tilapia will happily munch on any duckweed that is available. Some hobbyists have found that some barbs, mollies, and tetras also eat duckweed, though there is a rather large chance that they won’t touch them at all!

In all, if you don’t want to have your aquarium or pond covered in a mat of green, then it’s best to avoid duckweed entirely. Manual removal is still the best method for total eradication but will take some time and can become frustrating.

Conclusion

Duckweed is the common name for several types of flowering plants. It is known for being able to quickly take up nutrients and stabilize aquariums or ponds with algae problems. However, duckweed growth rate can become unmanageable very fast, and most hobbyists do not want to have to deal with removing clumps of this plant from their systems later on.

If you have any questions about duckweed care, how it helps aquariums or ponds, or have had experience removing duckweed from your own aquatic systems before, don’t hesitate to leave a comment below!

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