Herbivorous carp: what they really eat (and why they sometimes ignore your duckweed)

People often buy a grass carp with a specific idea in mind: that it will clear out the plants taking over the pond. A few weeks later, surprise: the elodea has disappeared, young water lily leaves have been nibbled… and the layer of duckweed is still there. The grass carp (Ctenopharyngodon idella) is indeed a herbivore, but a selective one: it chooses tender, submerged plants first and only turns to the rest afterward. Understanding its preferences helps avoid many disappointments.

A fish without a stomach that makes up for it through quantity

Like all carp, the grass carp has no stomach in the strict sense. It grinds its food with pharyngeal teeth located at the back of its throat, then passes it through a relatively short intestine for a plant eater. As a result, it extracts energy from plants rather poorly and must consume large quantities. In summer, a healthy fish can swallow a mass of plants each day that represents a significant proportion of its own weight, roughly speaking.

This anatomical detail has a direct consequence for the pond. Much of what is eaten emerges barely digested, in the form of nitrogen- and phosphorus-rich feces. In other words, the grass carp does not make plant matter disappear: it transforms it into dissolved fertilizer, available to microscopic algae. This is why a pond that suddenly loses its submerged plants often sees its water turn green soon afterward, a shift well documented in the ecology of shallow bodies of water.

What it chooses first, and what it leaves behind

Placed in front of a varied buffet, the grass carp does not eat randomly. Its preferences closely follow the tenderness of the plants and their water content. Here is the ranking most often observed in garden ponds and lakes.

This ranking explains why we so often see the same sequence: first oxygenating plants, then water lilies, and only then duckweed. If your specific goal is to control duckweed, you will therefore need to think differently; here, we are focusing on the diet itself.

Appetite follows the thermometer

Grass carp originate from the great rivers of China and the Russian Far East, where summers are hot. Their metabolism, like that of all fish, depends directly on water temperature. Below around 10 degrees, they eat very little and usually remain motionless near the bottom. Between 15 and 20 °C, they gradually start grazing again. It is above 20 degrees that they become truly voracious, so July and August are when they cause the most damage or provide the most benefits, depending on your perspective.

Practical consequence: a grass carp introduced in spring produces no visible results for weeks, and in Belgium or northern France, the period of real activity is limited to a few months a year. Plants therefore have the whole spring to gain ground, which is not necessarily a bad thing.

Young and adult fish do not eat the same things

Fry and very young grass carp begin their lives like most cyprinids: zooplankton, small insect larvae, then increasing amounts of plant matter as they grow. Grass carp sold at garden centers, generally measuring around 15 to 30 centimeters, are already largely herbivorous. In fact, these medium-sized fish are the most effective at eating fine vegetation such as duckweed, because their mouths are adapted to these small food items. Very large specimens often prefer more substantial masses of submerged plants.

Also note: in ponds and lakes in our regions, grass carp practically never reproduce. Their eggs need long stretches of flowing river to drift and develop. So you do not risk a population explosion, but each fish that is removed or dies must be replaced if you rely on it.

Should it be fed in addition to plants?

In a naturally vegetated pond, grass carp find plenty to eat from April to October. In an ornamental pond, it is a different story: once the tender plants have been consumed, it turns to whatever remains, including your ornamental plantings. Two common situations arise.

First case: you want it to retain an appetite for troublesome plants. In that case, limit pellets to the bare minimum needed for the other fish, and distribute them quickly so it does not get too much of them. A grass carp that is well fed on pellets becomes lazy.

Second case: it has already cleared everything out and you want to protect what remains. In that case, offer it plant-based supplements: untreated lettuce leaves, a small amount of untreated fresh grass, duckweed collected from another pond (while avoiding the introduction of unwanted species), and food rich in plant-based ingredients such as wheat germ. Remove anything uneaten after a few hours.

How its diet affects water quality

Because it turns plants into waste, grass carp moves nutrients from the “vegetation” compartment to the “water” compartment. In a large, well-balanced pond, this effect is diluted. In a pond of a few cubic metres, it is noticeable: higher nitrate levels, cloudier water, and more frequent suspended algae, especially if filtration is only just adequate. A few simple measures can limit this effect:

• test the water more often during the warm months, when the fish's appetite is at its peak;
• keep areas of protected plants (wire-mesh basket, area separated by a net) so the pond retains submerged vegetation;
• avoid keeping grass carp and a high koi population together in a small volume;
• harvest excess plants with a net rather than letting them all be consumed: the nutrients then leave the pond instead of returning to it.

What not to give it

Bread, leftovers, and treated plant matter (grass treated with weed killer, unwashed lettuce, houseplants) have no place in a pond. Bread swells, ferments, and degrades the water; pesticide residues can be toxic to all fish. Also beware of large amounts of grass clippings thrown into the pond: they decompose quickly and consume oxygen. Finally, never release a grass carp into a watercourse or public pond: its introduction into open waters is regulated, and its effects on aquatic vegetation can be severe.

Also read

To choose submerged plants that will regrow after your herbivorous fish have passed through: Discover our list of oxygenating plants for ponds / water features.

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Frequently asked questions

— Does a grass carp eat filamentous algae?

Very little. It shows interest only when more tender plants are unavailable, and it cannot solve a filamentous algae problem on its own.

— Why doesn't my grass carp touch the duckweed?

Because it finds better food elsewhere: tender submerged plants, water lily leaves, or pellets. It turns to duckweed only once these foods are depleted, and only in warm water.

— Can you feed lettuce to a grass carp?

Yes, untreated lettuce or grass, well rinsed, in small quantities. Remove leftovers after a few hours to avoid polluting the water.

— Does a grass carp eat in winter?

Hardly at all. In cold water, its metabolism slows considerably; it resumes feeding in spring and reaches its peak in midsummer.

In summary

• The grass carp is selective: tender submerged plants first, duckweed and filamentous algae much later.
• Its appetite depends on temperature: low in cold water, at its peak in summer.
• It digests plant matter poorly and releases much of the nutrients back into the water.
• In a small pond, limit pellets if it needs to forage, or supplement with plant matter if it has eaten everything.
• Protect some of the submerged plants to prevent a shift toward green water.

Last updated: September 29, 2026.