Rootless duckweed

At less than half a millimetre wide, this is the smallest flowering plant in the aquarium trade — and it doubles in mass faster than almost anything else. Wolffia arrhiza lacks roots entirely; it floats as tiny green spheres on the water surface, each one a complete plant that buds off daughters continuously.

It is not a plant anyone buys deliberately. It arrives as a hitchhiker on other floating plants, establishes within days, and covers the surface unless removed regularly. In a tank with still water and moderate light it forms a solid green mat that blocks light from below and reduces gas exchange at the surface.

That said, it has legitimate uses. Fry tanks benefit from the cover and the microfauna that live among the fronds. It pulls ammonia and nitrate from the water column aggressively, which can help during cycling or in heavily stocked tanks. And some killifish and livebearer species scatter eggs among floating plants as part of their natural breeding behaviour.

Removal is the main maintenance task — a single scoop takes out most of it, but a few remaining spheres repopulate the surface within days. Temperatures from 20 to 28 °C and pH 5 to 8 cover its range. It is not available from most aquarium shops intentionally but appears through plant trades and pond sources.