Common duckweed

Common duckweed divides so fast that a single plant can cover the surface of a small tank within weeks. Each frond is a flat, oval leaf 2 to 4 mm across with a single short root hanging into the water. It is the simplest aquatic plant in cultivation and one of the most controversial — aquarists either use it deliberately as a nutrient remover or spend years trying to eradicate it.

The plant absorbs ammonia and nitrate directly from the water column, which makes it effective at controlling algae during a tank cycle or in heavily stocked systems. However, a full surface mat blocks light from plants below and reduces gas exchange. The solution is regular removal: skimming off most of the surface cover each week keeps it in check.

It grows across an unusually wide range of conditions — pH 5 to 9, soft to very hard water, and temperatures from cool to tropical — and needs no CO₂ or fertiliser. Moderate light is enough for steady growth. Any light below that slows it but rarely stops it.

In outdoor ponds Lemna minor is winter hardy in temperate climates and survives near-freezing temperatures. Propagation is purely vegetative: each frond buds off daughter fronds, and a fragment left in the tank repopulates within days. Do not release it into local waterways — it is invasive in many regions outside its natural range.