Duckweed is a member of the Araceae family of angiosperms (flowering plants). They are the smallest representatives of flowering plants in nature.
They are quite small and buoyant, varying in size from 0.3 (Water Meal: Wolffia spp.) to 100 mm (Giant Duckweed: Spirodela polyrhiza) in diameter along the longest axis. There are 30 different species currently identified.
Duckweed is found in eutrophic (nutritionally-fertile) freshwater aquatic environments, can be seen typically forming a continuous mat of plants floating on the water surface, and ecologically serve as an important high-protein nutrient source for waterfowl. The mat-like covering over the water surface provides useful shading effects in exposed bodies of water and serves to quite effectively control pond and lake water temperatures.
This is of great benefit to fish and other aquatic organisms since dissolved oxygen levels are always inversely proportional to water temperature.
Just think about a cold bottle of carbonated cola. When you uncap it, very little gas escapes, whereas when the cola is hot, a sometimes violent eruption of gas can leave your countertop (or clothes) covered with a sticky liquid.
Duckweed reproduces through vegetative propagation (asexual budding) from an enclosed meristem at the base of the frond. They are classified as angiosperms since three extremely small flowers can be produced periodically, though which sexual reproduction occurs.
Flowers consist of two stamens and a pistil forming a simple flower, which is regarded by most botanists as a form of pseudanthium (structurally-reduced flower). Whether this inflorescence truly constitutes a primitive flower or has devolved from a more complex flower is currently a matter of debate.
Range expansion in duckweed typically occurs though water egress to streams from eutrophic bodies of water, but is also known to commonly occur through allochoric transport by animals after becoming attached to skin or feathers.
The distribution of duckweed is nearly global: North America, South America, Africa, Europe, Australia, and Central and Southeast Asia. The only continent in which duckweed has yet to be found is in Antarctica, for the obvious reasons of no available surface waters.
Benefits of Duckweed in Your Aquarium
Apart from duckweed giving your aquarium that natural, pond-like aesthetic appearance, there are several advantages to growing duckweed that will benefit the animal life in your tank. As part of a planted tank, duckweed performs many of the functions of other aquatic plants.
Let us review them.
Duckweed, like other aquatic plants, help to chemically stabilize the conditions in your tank by taking up byproducts from waste produced by vertebrate and invertebrate animals in your aquarium. Nitrates, nitrites, phosphates, and a variety of other chemicals and some toxins are consumed by duckweed as growth nutrients.
Duckweed can be used very effectively as part of a nitrogen and phosphorus management strategy in your aquarium.
Prevents Growth of Algae
A contiguous mat of duckweed in your tank will do a great job in limiting the penetration of sunlight and lamplight into deeper layers in the tank. This restriction has potential adverse effects on algal growth.
If you are using duckweed with other aquatic plants, however, be sure to use species that require low light levels or have been allowed to extend their fronds to the water surface. Otherwise, you can negatively impact the growth of the other aquatic plants in your tank.
Reduces Surface Evaporation
If you find that tank evaporation is an issue where you live, for example, in arid regions such as the southwestern US or far north, duckweed mats are able to reduce water loss through surface evaporation through light shading and limiting the area of open water.
Free Fish Food
Several species of common aquarium fish will readily eat duckweed for its high protein content. Most small fish species will consume duckweed as well as cichlids, goldfish, and tilapia.
However, duckweed is not advised for use with some fish species such as large cichlids, large goldfish, common plecos, and Ameca splendens (butterfly googeid).
Providing duckweed as cover can go a long way in reducing tank stress in your fish. Keep in mind that any sizeable aquarium you have seen will likely contain arched rock formations, logs (albeit ceramic), and other structures for fish to loiter in as cover.
This substantially reduces stress on fish by providing refuge from predators, nesting sites, and hide-outs for males that are not yet large enough to compete for mates. Duckweed can function in much the same manner by reducing the ability of predators above the water surface from seeing their targets (we know there are no predators hovering over your fish tank, but the fish do not know that).
As an added benefit, duckweed provides excellent cover for fry to prevent them from being poached by reproductively-competing adults.
Some Downsides of Using Duckweed
Smaller species of duckweed, if it overgrows, can get caught in the filter recirculation and require you to periodically remove the plants to prevent a reduction in water flow. You are likely cleaning your filters weekly, however, so this does not pose a problem for most aquarium enthusiasts.
Best Species of Duckweed to Use in Your Aquarium
Common duckweed (Leman minor), also known as lesser duckweed, is a common choice for aquarium enthusiasts. They often will mix this species in with a larger species, such as giant duckweed (Spirodela polyrhiza) for a more interesting appearance.
How to Care for Duckweed
The great thing about duckweed is that it grows fast, doubling in number after about 16-48 hours, depending on the species. It is an extremely tolerant plant and does not have very specific growth requirements.
Duckweed can grow in hard or soft water, and thrives in bright light, but will grow in low light conditions as well. If you want to maximize its growth, provide full-spectrum light from an aquarium lamp or direct exposure to sunlight.
Growth requirements for pH are broad (6.5-7.5) and duckweed can even survive in alkaline water up to about pH 9. Trace mineral supplements can also help duckweed flourish and there are several formulations that are mixed for the needs of duckweed and other aquatic plants.