Duckweed fern

Azolla filiculoides is the largest of the mosquito-ferns and one of the fastest-growing floating plants in the aquarium. It forms dense mats of overlapping, scale-like leaves on the water surface, and under strong light the entire colony turns a reddish-bronze colour. The red pigment is a protective response to high light, not a deficiency.

Each 'leaf' is actually a small, two-lobed frond that traps air and keeps the plant buoyant. Tiny roots hang from each frond into the water column. The plant doubles in mass within days under moderate to strong light, and a small cluster becomes a complete surface cover within two weeks.

It pulls ammonia and nitrate from the water aggressively, which makes it useful during cycling or in heavily stocked tanks. However, a full surface mat blocks light from plants below and reduces gas exchange at the surface, so regular removal is needed. A weekly scoop that takes out most of the surface cover is the standard maintenance routine.

It does not need CO₂ and grows across a wide temperature range from 18 to 30 °C and pH 5 to 8. In many regions Azolla filiculoides is invasive — never release it into local waterways. Propagation is by simple fragmentation: each frond that breaks off becomes a new plant.