Grass carp, koi, goldfish, tilapia, and common carp all eat duckweed readily. These are the most well-studied duckweed consumers, and each one approaches the tiny floating plant a bit differently depending on its size, appetite, and natural diet. Whether you’re trying to control duckweed in a pond or looking for a nutritious supplement for your fish, the species you choose matters.
Grass Carp
Grass carp are the go-to biological control for unwanted aquatic vegetation, including duckweed. They’re voracious herbivores that prefer soft, tender plants sitting at or below the water’s surface, which makes duckweed an easy target. The University of Arkansas recommends stocking 5 to 10 grass carp per acre for moderate plant coverage, but ponds with more than 50 percent coverage may need 20 or more per acre. Even at higher stocking rates, expect one to two years before they bring the duckweed under control.
One thing to keep in mind: grass carp aren’t picky eaters. Once they clear the duckweed, they’ll move on to other submerged plants you might actually want to keep. They also grow large, reaching 20 pounds or more, so they’re a pond solution, not an aquarium one.
Koi and Common Carp
Koi are a domesticated variety of common carp, and both will graze on duckweed floating on the surface. Research on common carp fed duckweed diets found that adding duckweed to their feed actually increased their protein content, lipid levels, amino acid profiles, and healthy omega-3 fatty acids. Their digestive enzyme activity also ramped up in proportion to how much duckweed was in the diet, suggesting their systems adapt well to it.
For pond owners, koi are a more manageable choice than grass carp. They won’t eliminate duckweed as aggressively, but a well-stocked koi pond can keep growth in check, especially during warmer months when the fish are most active. Since duckweed can double its mass in under 48 hours under ideal conditions, a small number of koi in a large pond won’t keep pace. You’ll need a healthy population relative to your pond size.
Goldfish
Goldfish are surprisingly effective duckweed grazers for their size. In feeding trials where goldfish fingerlings were given nothing but duckweed, they showed the highest growth rate and best feed conversion compared to other plant-based diets. Survival was 100 percent. Goldfish are omnivores that naturally nibble on plant matter, and duckweed’s small leaf size makes it easy for them to eat whole.
If you’re managing duckweed in a small backyard pond or want to offer a fresh green supplement in an aquarium, goldfish are a practical option. They won’t wipe out a thick duckweed mat in a large pond, but in contained spaces they can keep it under control while getting a nutritional boost.
Tilapia
Tilapia are one of the most studied fish in duckweed research because they’re widely farmed around the world. When fed duckweed alone, tilapia achieve a feed conversion ratio of 1:1, meaning they convert the plant into body mass very efficiently. However, their overall growth rate on a duckweed-only diet is slow, roughly 0.67 percent of body mass per day. When duckweed is combined with a standard pellet feed, growth rates double.
Dried duckweed can replace up to 50 percent of commercial tilapia feed for at least 50 days without reducing growth. Fresh duckweed is less efficient as a sole food source because it’s about 96 percent water, and the air pockets in its tiny leaves (which help it float) limit how much the fish can physically eat in a day. Drying it concentrates the nutrients and removes those barriers. For anyone raising tilapia, duckweed works best as a supplement rather than the entire diet.
Why Duckweed Is Nutritious for Fish
Duckweed punches above its weight nutritionally. Its protein content ranges from 20 to 30 percent of dry weight, which is high for a plant and comparable to some legumes. It contains vitamins A, B, and C along with minerals like phosphorus, potassium, and magnesium. It also delivers essential amino acids that support muscle growth and immune function in fish.
What sets duckweed apart from commercial feed is its flavonoid content. These are natural plant compounds with antioxidant properties. Duckweed contains about 158 milligrams of total flavonoids per 100 grams, including compounds that help reduce oxidative stress, support immune function, and improve gut health. Studies on grass carp fed a diet that included 25 percent duckweed showed consistently higher immune enzyme activity across multiple tissues compared to fish on commercial feed alone.
Fresh vs. Dried Duckweed
Fresh duckweed is what most pond fish encounter naturally, and they’ll happily graze on it as it floats by. The challenge is that fresh duckweed is almost entirely water. Fish have to eat a large volume of it to get meaningful nutrition, which limits how fast they grow on a fresh duckweed-only diet.
Dried duckweed concentrates the protein and nutrients into a denser package. In tilapia trials, fish grew better on dried duckweed than on fresh, though dried duckweed alone still couldn’t match commercial feed. The sweet spot in most studies is a blended approach: commercial feed mixed with dried duckweed at roughly a 50/50 ratio by dry weight. For aquarium keepers, tossing a small handful of fresh duckweed into the tank is the simplest approach, since the fish will graze on it throughout the day.
Matching Fish to Your Situation
Your best choice depends on what you’re trying to accomplish. If you have a large pond overrun with duckweed, grass carp are the most effective option, though they take time and will eat other plants too. Koi work well for decorative ponds where you want moderate duckweed control without eliminating every aquatic plant in sight. Goldfish are ideal for small ponds and aquariums. Tilapia make sense if you’re in a warm climate and raising fish for food.
Keep in mind that some duckweed species contain oxalic acid, which can make the plants less palatable. If your fish seem uninterested in the duckweed you’re offering, the specific species of duckweed could be the issue. The most commonly eaten varieties belong to the Lemna and Spirodela genera, both of which have performed well in feeding studies across multiple fish species.
Because duckweed grows so rapidly, doubling in mass every two days under good conditions, it’s entirely possible to grow your own supply in a separate container and harvest it regularly for your fish. A shallow tub with some fertilizer and sunlight will produce a steady crop, giving you a free, renewable source of high-protein fish food.