Subclass Alismatidae ( Alismatidae )
Order Alismatales ( Alismatales )
Family Araceae ( Araceae )
Genus Duckweed — Lemna.
In all species of these plants, the stem is transformed into a flat or slightly convex plate, from the lower surface of which simple thread-like roots extend. Leaves are represented by tiny scales. The stem has 1-2 marginal pockets in which daughter plants develop. In an aquarium, most of these plants rapidly grow and cover the entire water surface, while some form patches in the water column or on the surface. Aquarists often use these plants as a filter for aquariums set up in bright rooms. Duckweed cleans the water well and enriches it with oxygen.
Common Duckweed — Lemna minor.
“1” (Eng. Common Duckweed, Lesser Duckweed). It is distributed practically on all continents in water bodies with standing or slow-flowing water. This tiny perennial representative of the higher flora floats on the surface and in the warm season covers the entire area of the water body with a solid carpet. The plant floating on the water surface consists of separate oval light green plates up to 5 mm in size with a single root. On one side, the plate branches, and from its underside, a thin translucent root extends, the tip of which is enclosed in a so-called pocket. The role of the roots is purely mechanical – to help maintain the balance of the plate. The roots reach a length of 6-8 mm. Intensive growth indicates a normal biological state of the aquarium.
“2” The water temperature at which duckweed can exist ranges in a very wide range – from 15 to 30°C. Water hardness and active reaction (pH) do not matter for it. The only requirement is the brightness of lighting in the aquarium. It is also important that duckweed, like other members of the family, is an excellent natural chemical filter: it removes toxic compounds from the water, including heavy metals. True, with their excess, the plant tissues are destroyed, so if duckweed in your aquarium “suddenly” begins to die for no apparent reason, you need to check the water quality.
“5” According to the prevailing opinion, duckweed is appropriate in a decorative water body only if there are no other, more beautiful and useful plants floating on the water surface. Growing, it releases a large amount of oxygen and is a good water purifier, clearing water from suspended particles. Duckweed is used to shade plants that fear bright light, acting as a light filter, and as a vitamin supplement to the diet of many fish.
Duckweed is a kind of indicator of water quality and habitat. Due to its rapid growth, it is a good plant for absorbing nitrates and phosphates in the aquarium. If duckweed and Riccia grow in the aquarium, it means there is no cause for concern: a normal biological balance has been established in the aquarium. Under favorable conditions, this duckweed can form dense and large colonies in the water column. If duckweed has enough light in the aquarium, it lives there all year round, forming a carpet on the surface with roots hanging down, among which fry successfully hide. With strong growth, duckweed should be periodically thinned out.
To use as food, it is dried on a baking sheet, ground into powder, and added to feeds. It is eagerly eaten by goldfish. In aquariums, it is often regarded as a weed that is difficult to get rid of. Duckweed often clogs the filter and thus prevents the normal functioning of the water purification system in the aquarium. To get rid of it, you need to collect it from the surface with a net and turn off the top lighting for 4-7 days, temporarily creating side lighting for the aquarium. Usually, this leads to the death of duckweed. If the plant was subjected to frost in autumn, it dies and sinks to the bottom. Young shoot buds do not suffer from frost; they overwinter at the bottom and rise to the surface in spring.
“6” Duckweed is distinguished by amazing fertility. It reproduces mainly by offshoots that extend from the plate and eventually become independent plants. The shoots remain connected to the mother plant for a long time and form multi-plate groups.
Gibberous Duckweed — Lemna gibba.
In the wild, it can be seen in water bodies of Western or Central Europe. Small plants floating on the water surface. It is found much less often than lesser duckweed. A small plant floating on the water surface. The leaves are very small, convex below, filled with air chambers that provide buoyancy. Leaves are single, oval, bright green, sometimes with red spots, occasionally red underneath. Stem length up to 1 mm. Its leafless stem is a slightly concave, thin, bright green round plate up to 8 mm in diameter, from which a single thin translucent root extends.
Daughter shoots arise from two lateral pockets of the mother plant. Maintenance conditions are similar to lesser duckweed. This duckweed loves light and is capable of vigorous reproduction. Gibberous duckweed is also a good indicator. If the plant does not develop well, it means the water is poor in nutrients and needs fertilization. For the winter, this plant forms winter buds that allow the plant to survive and produce leaves in the spring.
Ivy-leaved Duckweed — Lemna trisulca.
“1” In natural conditions, duckweed lives in stagnant water bodies of all temperate climate zones. A very beautiful plant floating in the water column, consisting of light green translucent leaflets connected in threes, about 1 cm long. It forms thickets in the water in the form of an openwork ball, in which fry of many fish find food and shelter. It lives in the water column, and during the flowering period, it floats to the surface. Its plates are translucent, often three-lobed, light green, lanceolate, up to 1.25 cm long. Leaves grow in bunches of three or more. Shoots remain connected to the mother plant for a long time and form multi-plate groups.
This duckweed differs from lesser duckweed primarily in the shape of the leaf plate, seemingly divided into three lobes — hence the name of the plant. The leaves of lesser duckweed float on the very surface of the water, while the bulk of ivy-leaved duckweed is not on the surface, but in the water itself. Its leaves are light green, translucent, connected in threes, and reach a length of 1 cm.
“2” The plant is best suited for a moderately warm aquarium. The optimal maintenance temperature is 18-22°C. The water should be soft (total hardness preferably less than 8°) with a neutral or slightly acidic reaction. In harder water with a slightly alkaline reaction, the plant is less durable. The water in the aquarium must be very clean, as the settling of turbidity particles on the plant leaves disrupts their nutrition and quickly leads to their death. Lighting during the growth of duckweed should be very strong, but diffused.
“5” Under favorable growth conditions, it can form dense and large patches in the water column, and during the flowering period – on the surface. Ivy-leaved duckweed helps saturate the water with oxygen. The thickets of ivy-leaved duckweed formed in the water serve as a spawning ground and shelter for many small fish. In natural conditions, duckweed rather quickly forms thickets and covers the entire surface of the water body. The same will happen in the aquarium if you do not monitor the plant. Therefore, to avoid strong darkening of the aquarium, it is necessary to regularly thin out the thickets.
It has a pronounced growth seasonality. In winter, duckweed must be kept in separate containers at a water temperature not exceeding 12°C. Until December, duckweed is kept in the aquarium, then it is placed in a special shallow dish and stored under low light and a temperature of 5-12°C. In March, the plant can be transferred back to a warm aquarium.
“6” It reproduces by offshoots that remain connected to the mother plant for a long time. Duckweed is very easy to propagate by separating the most insignificant part from a large thicket. Under conditions comfortable for the plant, a new plant can form from just a few leaves.
Gymnocoronis spilanthoides, Shinnersia rivularis.
Cryptocoryne wendtii, Cryptocoryne willisii, lucens, Cryptocoryne scurrulis. 
