Duckweed

Overview

Duckweed (Lemna minor) is the tiniest common floating plant — single round green fronds a few millimetres across that carpet the surface and multiply at an astonishing rate. That speed is its whole story: it is one of the best nutrient sponges and natural algae fighters you can add, and many fish graze on it, but it is also invasive, unstoppable and nearly impossible to remove once established. Duckweed is genuinely useful, but only add it with your eyes open.

Planting & placement

There is no planting — duckweed simply floats on the surface and spreads on its own. That is precisely the problem: filter flow scatters it across the whole tank, it sticks to nets, hands, plants and equipment, and it travels between tanks on the smallest droplet. If you want it contained, turn off surface agitation so it gathers in one area you can skim. It needs no substrate and no care to establish; the difficulty is all in controlling it, not growing it. Leave open surface for gas exchange and feeding.

Light, CO2 & ferts

Duckweed grows under low to high light and needs no CO2 — brighter light just makes it multiply even faster. It feeds voraciously from the water column, which is exactly why it is such an effective nutrient and nitrate sponge; in most tanks it needs no dedicated fertilizer and thrives on fish waste alone. Its appetite is what lets it outcompete and shade out algae.

Propagation & problems

Propagation needs no effort at all — each frond buds off new fronds continuously, doubling the population in days. That is the entire challenge: control, not growth. The only real problems are its invasiveness and the way a thick mat blocks light and gas exchange, so skim it constantly and keep some surface clear. Removing it is a war of attrition: scoop relentlessly, clean equipment between tanks, and accept you may never be fully rid of it. Used deliberately as a nutrient sponge in a tank where its spread does not matter, though, duckweed is unmatched.