Drying Duckweed Into Fish Food: What the Research Shows

The short answer

Yes. Duckweed has been fed to farmed fish for decades, fresh and dried, and the research behind it is solid. What the research does not include is medaka. Nobody has tested duckweed meal in medaka or in any small ornamental fish like them, so what follows is the evidence for other fish, plus my best reasoning about what that means for a medaka tub. I have not run my own feeding trials, and I will not pretend I have.

What the research shows

• It works as a partial feed. A 2024 review of 58 studies covering 18 fish species found that duckweed at roughly 15 to 30 percent of the diet gave good growth and feed conversion with no survival penalty, whether it was fed live, dried, or pelleted.
• Giant duckweed specifically has been tested. Nile tilapia did fine when meal from Spirodela polyrhiza made up to 30 percent of their diet, and performance dropped at higher levels.
• Fresh duckweed fed grass carp well, tripling their weight in 50 days in one early trial.
• Koi did best at a small share. The best result was 10 percent duckweed replacing beef heart, though higher levels still supported growth.

Those are farm and pond fish with much bigger mouths and appetites than a medaka, which is exactly why I would treat the numbers as a reason to experiment gently, not as a recipe.

The quality depends on how you grew it

Duckweed is not one fixed food. Protein swings widely with the water it grew in: in one lagoon study, giant duckweed measured about 13 percent protein in nutrient-poor water and nearly 40 percent in nutrient-rich water. A clean ornamental tub is nutrient-poor by farm standards, so I expect the low end. That is my inference, not a measurement.

There are also natural compounds worth knowing about. Duckweed stores calcium oxalate crystals, which are harmful in large amounts, and growing it in low-calcium water is reported to reduce them. And one study flagged lower digestibility for Spirodela than for common duckweed in a pig model. That is one study in a different animal, but it is another reason to think of duckweed as a supplement and not a staple.

Would it help medaka?

Medaka are small surface feeders with tiny mouths. They will not eat duckweed fronds, and I have found no data on them eating it. The realistic roles are these.

• Living plants give shade, cover, and root surfaces for the tiny organisms medaka graze on. That is how I use duckweed, and it is the whole point of the plant in a tub.
• Dried and ground to a fine powder, it could be a small supplement mixed into a quality pellet. Whether medaka benefit is untested, so I would start with a pinch and watch the water and the fish.

It is not a replacement for a good medaka pellet, and I would not build a diet around it.

Safety comes first

• Use a clean source only. Duckweed soaks up heavy metals, pesticides, and other contaminants from the water it grows in, which is well documented in wastewater systems. Never harvest from ditches, retention ponds, roadsides, or anywhere that might carry runoff.
• Growing your own is the safe route. Duckweed from a clean tub filled with clean water, with no pesticide exposure, is low-risk by that same logic. No study addresses that exact case, which is why I am calling it reasoning and not a guarantee.
• Skip bloom water. Do not harvest from water with an algae bloom.
• Hitchhikers are undocumented. I found nothing on parasites or snail eggs traveling with duckweed, so rinse well and dry it thoroughly.

How to dry it

Fresh duckweed is 92 to 95 percent water, so drying is most of the work. These are the documented numbers, with my notes on what is solid and what is hobby practice.

1. Harvest and rinse. Lift the plants out, rinse them gently, and let them drain well.
2. Spread them thin. A single thin layer on a tray or dehydrator sheet dries far faster than a pile.
3. Use low heat. One trial needed about 30 hours at 40°C (104°F) to take duckweed from 95 percent moisture to around 10 percent. A study on a related plant, Wolffia, got to under 3 percent moisture in 6 hours at 50°C (122°F) and found hotter drying degraded the nutrients. Older work found samples looked burned at 120°C (248°F) and above.
4. Be wary of sun drying. It can take one and a half to three times longer than an oven. In humid weather like a Carolina summer, a slow dry invites mold, though that last part is my reasoning, not a measured figure.
5. Check it is fully dry. It should be brittle and crisp, not leathery. That test is a common hobby rule, not a lab standard.
6. Grind it fine. A blender or spice grinder gives you powder small enough for tiny mouths.
7. Store it airtight and dry, in small batches. I found no documented shelf life, so throw out anything that smells off or looks moldy.

Feeding it

If you try it, start with a small pinch mixed into your usual food, watch whether the fish take it, and keep an eye on water quality. The percentages in the research above are for other fish, and I would not carry them over to medaka. If your group does not seem interested, nothing is lost.

Other ways to use the extra

If drying feels like too much, a second tub is the easiest outlet, and so is a neighbor with a tub of their own, as long as the plant is legal in their state. Dry any leftovers before they go in the garden or the trash, since live floaters can regrow from fragments, and never dump them in a waterway.

Sources

• Minich and Michael 2024, a review of duckweed in fish feeds (58 studies)
• FAO: Duckweed, a tiny aquatic plant with enormous potential (composition, harvest, contaminants)
• Feedipedia: duckweeds (drying, oxalate, inclusion levels)
• Jayarathne et al. 2013, duckweed in koi diets
• Evaluation of the biochemical composition of greater duckweed, Spirodela polyrhiza
• Prosridee et al. 2023, drying temperatures for Wolffia
• Lawson 1973, drying duckweed (LSU thesis)
• Review of heavy metals, pathogens, and xenobiotics in waste-grown biomass

Common questions

Can you dry duckweed and feed it to fish?

Yes. Duckweed is fed to many farmed fish, fresh or dried, and giant duckweed meal worked as up to 30 percent of a tilapia diet. It has not been tested in medaka, so treat it as an experiment.

How do you dry duckweed for fish food?

Rinse and drain it, spread it in a thin layer, and dry it at low heat, about 40°C (104°F) for around 30 hours in one trial, until it is brittle. Grind it to a fine powder and store it airtight.

Is it safe to feed fish duckweed?

From a clean source it is considered low-risk, but duckweed absorbs metals and pesticides from polluted water, so never harvest from ditches, ponds with runoff, or bloom water. Growing your own in a clean tub is the safe route.

Can medaka eat duckweed?

I found no research on it. Their mouths are small, so they mostly use living duckweed for shade and cover, and a finely ground dried powder is the only realistic way to feed it.

How much protein is in duckweed?

It varies with the water it grows in. Giant duckweed measured about 13 percent protein in nutrient-poor water and nearly 40 percent in nutrient-rich water in one study.