Common Duckweed: absorbs nitrate, but turns into a pest

Common duckweed is the smallest and most prolific floating plant in the aquarium: tiny round leaves two or three millimetres across that float at the surface with a single root hanging down. It has two sides. The good: it takes up nitrate and phosphate very quickly, provides shade and holds algae back. The bad: it doubles in a few days, covers the whole surface, gets into the filter and is almost impossible to eradicate once it is in. That is why many keepers treat it as a pest. It is native to Europe, but that is no reason to dump surplus from an aquarium outdoors.

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