Duckweed: friend, foe and nutrient sponge
Lemna minor
The tiny floating plant hobbyists both love and dread.
Where it comes from
Lemna minor is one of the most widespread flowering plants on Earth, found on still and slow fresh water across temperate and tropical regions worldwide. Each plant is little more than a floating leaf-like frond a few millimetres across with a single fine root beneath.
It reproduces by budding, not seed, which is why a handful can blanket a surface in weeks — and why it so often arrives uninvited on new plants or with fish.
The case for duckweed
Duckweed is a phenomenal nutrient sponge. Because it draws CO2 from the air and nutrients straight from the water, it grows fast and strips ammonia and nitrate aggressively — a genuine tool against algae in an overstocked tank. It also shades the tank, offers cover for fry and shrimp, and is eaten readily by goldfish, many cichlids and other herbivores as a free, protein-rich food.
How to keep it
The challenge with duckweed is never keeping it alive — it is keeping it in check. It asks for nothing but light and will grow in almost any tank. The one thing it dislikes is surface turbulence, which pushes fronds together and traps them against the glass.
| Parameter | Value |
|---|---|
| Lighting | Low to high — grows under almost any light |
| CO2 | Not required; draws CO2 from the air |
| Temperature | 15–30 °C |
| pH | 6.5–7.5 |
| Hardness | Soft to hard |
| Fertiliser | Not needed; feeds on tank waste |
| Substrate | None — floats |
| Growth rate | Very fast |
| Placement | Floating |
| Difficulty | Easy |
Living with it
Skim it off by the netful every week to keep numbers down and to physically export the nutrients it has locked up. A gentle surface current will corral it to one corner for easy removal. If you want it gone entirely, be warned: a single frond left behind will repopulate the tank, so removal has to be relentless and complete.
What goes wrong
Left unmanaged, a thick mat blocks light to plants below and slows gas exchange at the surface, which can lower oxygen at night. Keep it thinned. If it suddenly turns white or yellow and dies back, that is usually a lack of light beneath an overcrowded mat, or a large swing in water chemistry.
More plants in this series
- Amazon frogbit (Limnobium laevigatum) — the larger, tidier floater
- Red root floaters (Phyllanthus fluitans) — the colourful nano floater
- Water lettuce (Pistia stratiotes) — the big-rooted surface floater
- Guppy grass (Najas guadalupensis) — the fast fry-cover exporter