Duckweed as free fish feed

Duckweed doubles its biomass every 2-4 days, contains 25-45% protein, and most aquaponics fish eat it readily. How to grow it and how much it actually offsets your feed bill.

Duckweed is a tiny floating plant that covers still water surfaces in a bright green mat. It's considered a nuisance in natural waterways because it blocks light and depletes oxygen. In aquaponics, those same properties make it one of the most useful companion organisms you can add to the system. It grows fast enough to produce harvestable biomass every few days, it has a protein content comparable to soybean meal, and fish eat it enthusiastically.

It won't replace commercial feed entirely, but it can reduce your feed costs by 20-40% while improving fish health through dietary variety.

Growth rate

Duckweed (Lemna minor, Lemna gibba, Spirodela polyrhiza, and related species) is one of the fastest-growing plants on earth. Under favorable conditions (warm water, adequate nutrients, sufficient light), it can double its biomass every 2-4 days. A small patch covering a 30 cm square can fill a 1-square-meter tray within two weeks.

This rapid reproduction is what makes it viable as a feed crop. You don't plant duckweed, wait months, and harvest once. You skim off half the surface every few days and feed it directly to the fish. The remaining duckweed fills the space back in within days.

Growth rate depends on temperature (optimal at 20-30 C, minimal below 10 C), light (full sunlight or strong artificial light), and nutrient availability. In an aquaponics context, the nutrient supply is built in: system water rich in nitrate and other dissolved nutrients feeds the duckweed continuously.

Nutritional profile

Duckweed protein content varies from 25% to 45% of dry weight depending on species, growing conditions, and nutrient availability. When grown in nutrient-rich water (which aquaponics water is), protein content is at the higher end of this range. For comparison, soybean meal (the primary protein source in most commercial fish feed) contains about 44-48% protein.

Beyond protein, duckweed contains essential amino acids, carotenoid pigments (which enhance fish coloration), vitamins A and B complex, and trace minerals absorbed from the growing water. Studies have shown that tilapia fed a diet supplemented with 20-30% duckweed by weight show comparable growth to tilapia fed 100% commercial pellets, with the added benefit of improved gut health from the natural fiber content.

The main limitation: methionine is often the first limiting amino acid in duckweed, as in most plant proteins, so its amino acid profile doesn't fully match fishmeal-based commercial feed. This is why it supplements but doesn't fully replace pellet feed, especially for fast-growing species like tilapia and trout that need complete amino acid profiles for optimal growth.

Which fish eat it

Tilapia: Natural duckweed grazers. Tilapia eat duckweed eagerly and are the best species for duckweed supplementation. In pond aquaculture, tilapia are sometimes grown on diets that are 50%+ duckweed.

Goldfish: Omnivores that eat duckweed readily. In a goldfish aquaponics system, duckweed grown in a separate container and added to the tank every few days provides both nutrition and enrichment.

Koi and carp: Enthusiastic duckweed eaters. Koi ponds often struggle to keep duckweed alive because the fish consume it as fast as it grows.

Catfish: Will eat duckweed but less enthusiastically than tilapia or goldfish. Catfish prefer animal-based protein and may need encouragement (mixing duckweed with pellets).

Trout: Not natural plant eaters. Trout are carnivorous and generally won't eat duckweed voluntarily. Not a good match for this supplementation strategy.

How to grow it

Separate container. Don't grow duckweed directly in the fish tank; the fish will eat it faster than it can grow. Instead, grow it in a separate shallow container (a plastic tub, a spare grow bed, a kiddie pool) filled with system water. The nutrients in the water feed the duckweed without any additional fertilization.

Conditions: Full sunlight or a grow light for 12-16 hours per day. Water depth of 10-20 cm is enough; duckweed floats on the surface and doesn't need depth. Still water, or very gentle flow. Strong currents push duckweed to one side and can submerge it. Temperature above 15 C for active growth.

Stocking the culture: Start with a small amount of duckweed from a pond, aquarium, or online supplier. It will fill the available surface within 1-2 weeks under good conditions.

Harvesting: Skim half the surface every 2-4 days using a fine mesh net or a plastic strainer. Feed the harvested duckweed directly to the fish. If you harvest more than the fish can eat immediately, store it in a bucket of water in a cool place for up to a week, or dry it for later use (though fresh is more palatable to most fish).

How much does it offset

A 1-square-meter duckweed growing area under good conditions produces roughly 50-100 grams of fresh duckweed per day (fresh weight; dry weight is about 5-10% of that). At 30-40% protein on a dry basis, that's roughly 1.5-4 grams of protein per day from one square meter.

A tilapia system with 10 kg of fish consuming 3% body weight daily needs 300 grams of feed per day, containing roughly 90-120 grams of protein (at 30-40% protein feed). Your square meter of duckweed provides about 2-4% of the total protein requirement. Not much.

To meaningfully offset feed costs (20-30% replacement), you'd need 5-10 square meters of duckweed growing area for a 10 kg fish stock. At this scale, duckweed growing starts to compete for space with the main grow beds.

The economics work best when: you have spare sunny space (a yard, patio, or unused area near the system), you're growing duckweed in system water that would otherwise be wasted during water changes, and you value the dietary variety and health benefits for the fish beyond pure protein math.

Cautions

Don't collect wild duckweed from ponds or ditches without quarantine. Wild duckweed can carry snail eggs, parasites, and contaminants from the collection site. Rinse thoroughly and grow in clean water for a week before feeding to fish.

Duckweed in the main system will clog pump intakes and block light to the water surface if it escapes containment. If some gets into the fish tank, the fish will eat it (good), but it can also spread to sumps and grow beds where it's unwanted.

Not a complete diet. Always provide commercial pellet feed as the base diet. Use duckweed as a supplement, not a replacement. Fish fed only duckweed grow more slowly and may develop nutritional deficiencies.

The fish-to-plant ratio calculator can help you determine feeding rates, and the duckweed offset can be factored into your total feed budget.