Duckweed in aquariums: good plant or pest? How to control it or get rid of it

Duckweed is a tiny floating plant that can be a real help in an aquarium: it soaks up nitrate and gives shy fish shade. But it doubles fast, and if it covers the whole surface it blocks light for other plants and slows the exchange of gases at the water's surface. Keep it to part of the surface and scoop out the extra every week, or plan on a careful, weeks-long effort to remove it completely.

Whatever you do, never tip duckweed, or tank water with duckweed in it, into a pond, drain or stream. It spreads easily outdoors and some duckweed species are invasive.

How to identify it

Common duckweed (Lemna minor): flat, oval, light green "leaves" (called fronds) about 2 to 4 mm (1/16 to 1/8 inch) long, often in clusters of two to five, each frond with a single thin root hanging down.
Dotted duckweed (Landoltia punctata): slightly longer, darker green fronds with 2 to 4 roots each. It is common in the aquarium and water-garden trade and has spread across much of the United States.
Giant duckweed (Spirodela polyrhiza): broader fronds, often with a purple-red underside and many roots.
Watermeal (Wolffia): even tinier, rootless green grains, like floating green sand.

Duckweed usually arrives as a hitchhiker on new plants or in bags of fish. One or two fronds are enough to start a carpet.

When it's a problem

Helpful: a loose patch covering a third to half of the surface. It uses up nitrate, competes with algae and gives fish cover and shade.
Becoming a problem: it covers most of the surface, plants below grow leggy or pale, and duckweed gets sucked into the filter.
Serious: a solid green mat. Rooted plants starve of light, surface gas exchange drops, and you may see fish hanging at the top. In outdoor ponds, a complete cover lasting a long time can cause oxygen crashes and fish deaths.
Emergency: if fish are gasping at the surface under a thick mat, scoop out most of it now, add an air stone, and test the water.

What to do right now

Scoop out the extra with a fine fish net or a small kitchen sieve. Leave a patch if you want to keep some.
Clear an open area around the filter outlet so the surface can move.
Add surface movement or an air stone if fish look short of breath.
Clean bits out of the filter intake and media.
Bin or compost removed duckweed. Don't flush it or tip it outside near water. Letting it dry out in the sun kills it quickly.

What to do next

Option 1: keep it, but control it

Corral it. Float a ring of airline tubing (joined into a circle with a connector) or a ready-made feeding ring to keep duckweed in one area. This keeps it away from the filter and leaves open water for gas exchange and feeding.
Thin it weekly. Net out a third to a half each week. Removing duckweed also removes the nitrate and phosphate it absorbed.
Turn up the surface flow a little. Duckweed prefers still water and grows less where the surface is rippling.
Don't over-fertilise. Duckweed thrives in nutrient-rich water. Less food and fertiliser means slower duckweed.

Option 2: get rid of it completely

This takes patience. One leftover frond can repopulate the tank in a few weeks.

Net it out every day for at least two weeks, then every few days for another month.
Lower the water level a few centimetres, wipe the glass at the waterline, and check under lids, around heaters and in plant leaves where fronds hide.
Check the filter: intake sponges, the inside of hang-on-back filters and the outlet are favourite hiding spots.
Rinse new plants under running water and check them one by one. Quarantine new plants in a bucket for a week or two so any hitchhikers show up there, not in your tank.
Use a net only for this job, and dry it out after.

Avoid chemical weed killers in an aquarium. The products used for duckweed in ponds are herbicides for outdoor water and can harm fish, shrimp and your other plants.

Do fish eat it?

Some do. Goldfish, many cichlids, silver dollars and some large plant-eating fish eat duckweed eagerly, and some keepers grow it as a high-protein fish food. Its protein content is often quoted at around 20 to 40%. Most small community fish, such as tetras, rasboras and guppies, only nibble it and won't keep it under control. Don't buy fish just to eat duckweed; they will eat your other plants too.

Cautions for other tank residents

Labyrinth fish (bettas, gouramis) need to gulp air at the surface. Always leave them clear surface space.
Bubble-nest builders often like some floating plants to anchor nests, so a small patch can help.
Shrimp and snails are fine with duckweed and may graze on it.
Rooted plants that need strong light will be the first to suffer under a mat.

Products that help

Fine-mesh net or small sieve: for scooping duckweed off the surface.
Floating plant ring or airline tubing ring: corrals duckweed into one area.
Air stone and air pump: keeps the surface moving and oxygen up when floating plants are thick.
Filter intake sponge (pre-filter): stops fronds being sucked into the filter.
Nitrate test kit: shows how much duckweed is helping, or whether you're feeding too much.

How to prevent it

Rinse and inspect every new plant, and quarantine plants if you don't want duckweed.
Don't add bag water from a shop or another tank into your aquarium.
Keep nutrients moderate: feed lightly and change water weekly.
If you keep it, thin it every week before it covers the surface.

Keep it out of ponds and waterways

Duckweeds travel easily on birds, boats, nets and buckets, and they can grow fast in quiet, nutrient-rich water. Some types are native in parts of North America, but others are not. The US Geological Survey, for example, tracks dotted duckweed as a non-native species thought to have spread largely through the aquarium and water-garden trade. Even native duckweed can smother a garden pond. Dispose of extra duckweed in the bin or compost (dried out first), never in a ditch, pond, drain or toilet. Never release tank water, plants or animals outdoors.

Why it happened

Duckweed almost always comes in on new plants, or in water from a shop or a friend's tank. It explodes in tanks with plenty of light, still surface water and lots of nitrate and phosphate. A sudden duckweed boom is a hint you may be overfeeding.

When to get help

Ask a local fish club or aquatic plant shop if you can't tell which floating plant you have, or if fish keep gasping after you've cleared the surface: that points to a water-quality or oxygen problem, so test the water. For ponds, contact your local conservation or invasive species authority before using any treatment, because rules on herbicides in outdoor water vary.

In the game

Multiple Tank Syndrome doesn't have a duckweed card yet.