Duckweed

Quick facts

Max height

1 cm

Growth rate

fast

Difficulty

beginner

Placement

floating

Propagation

fragmentation

Water parameters

Temperature

5–30°C

pH

5.0 to 9.0

Hardness

0 to 30 dGH

Cold water

tolerated (unheated setups)

Light and nutrients

Lighting

low

CO2

not needed

Substrate

floating

Feeding

feeds from the water column (use liquid fertilizer)

Substrate

What this plant roots into (or attaches to). The substrate affects both plant nutrition and water chemistry; see each linked page for full effects.

This plant feeds primarily from the water column, so substrate choice matters more for its fish-tank compatibility than for plant nutrition.

With fish

Plant-eating fish

will be eaten by mollies, silver dollars, large goldfish, and other plant-grazers

Diggers (corydoras, loaches)

fine - root system or attachment style handles it

Root-disturbing fish

tolerates fish that disturb roots

Habitat

Found on every continent except Antarctica. The family Lemnaceae includes several genera (Lemna, Spirodela, Wolffia) collectively called duckweed. The most common aquarium and aquaponics species is Lemna minor. These are the smallest flowering plants in the world; individual fronds are 1–5 mm across, consisting of a single flat green disc with one or more trailing roots. Duckweed floats on still or slow-moving water surfaces of ponds, ditches, marshes, and quiet river backwaters. Growth is exponential under favorable conditions: each frond buds daughter fronds continuously, and a population can double in mass every 2-3 days in warm, nutrient-rich water with good light. This explosive growth rate is the plant's defining characteristic and the source of both its utility and its infamy in aquarium keeping.

Outdoor pond use

This species transitions to outdoor ponds well, not just indoor aquariums.

Outdoor pond zones (USDA)

4 to 13 (winter low around -34°C or warmer)

Below the minimum zone, the plant won't overwinter outdoors but can still be grown seasonally and overwintered indoors. Several pond-friendly species (water hyacinth, water lettuce, parrot's feather) are regulated as noxious in some jurisdictions; check the legality data on the profile before releasing anything to an outdoor body of water.

Care notes

The fastest-growing aquatic plant available and a polarizing species among aquarium keepers. Some love duckweed for its nutrient-export capacity (it strips nitrate and phosphate from the water aggressively), its usefulness as live food for herbivorous fish and shrimp, and its role as shade for sensitive species. Others despise it because once introduced, it's nearly impossible to eradicate; it fragments into tiny pieces that persist through water changes, net sweeps, and even complete teardowns. A single overlooked frond restarts the population. In aquaponics, duckweed is a valuable supplemental feed: high in protein (25-45% dry weight), fast-growing, and harvestable continuously. Many tilapia, goldfish, and carp operations use duckweed as a portion of the fish diet. Requirements are minimal: any light above dim, any temperature from 5–35°C, any water chemistry. Growth is fastest in warm (25–30°C), nutrient-rich, still water with strong light. Surface agitation from filters slows duckweed growth and pushes it into corners; still water lets it cover the surface evenly. The main maintenance task is harvesting: skim excess duckweed with a net 2-3 times per week to prevent complete surface coverage that blocks light to submerged plants.

Further reading

• How to cycle a fish tank
• Water chemistry for fishkeepers: pH, GH, KH, and what they mean