Raising daphnia (Daphnia magna or Daphnia pulex): choosing the species, setting up a sustainable culture, and their role in keeping pond water clear

They are called water fleas, but they are tiny crustaceans that are among the most useful organisms in a body of water. Daphnia continuously filter microscopic algae from green water and provide exceptional live food for fry and small fish; when properly established in an outdoor tank, a culture can produce regular harvests from spring through autumn. You still need to choose the right species and avoid the few mistakes that can cause a culture to collapse within a week.

What is a daphnia?

Daphnia are planktonic crustaceans a few millimeters long, protected by a transparent shell. They move in small leaps thanks to their large antennae, hence their nickname water fleas. Their thoracic legs form a filter that captures microscopic algae, bacteria, and fine particles. A dense population can filter a large portion of a pond's volume in just a few days: this is one of the reasons for the "clear-water phase" observed in many bodies of water in spring, when daphnia gain the upper hand over phytoplankton.

Their secret to productivity: in good conditions, females reproduce without males through parthenogenesis and give birth to young every few days. When conditions deteriorate - overcrowding, lack of food, shorter days - males appear and females produce resting eggs, protected in a dark shell called an ephippium, capable of surviving freezing and drought.

Daphnia magna or Daphnia pulex?

For a first outdoor tank culture, Daphnia magna is generally the easiest and most cost-effective. If your goal is to feed very young fry, keep a culture of D. pulex alongside it.

Setting up a lasting culture

Container

A tank, tub or large basin holding 50 to 200 litres, made of food-grade plastic and placed in partial shade. Full sun makes the temperature rise too high in summer, while full shade prevents green water from developing. Avoid metal containers, as copper and zinc are highly toxic to daphnia.

Water

Clean rainwater or dechlorinated tap water that has been left to stand for a few days. A pH between 7 and 8.5 suits them. Daphnia are very sensitive to chlorine, soaps and residues from treatment products: never use a bucket that has contained detergents.

Food

The best food is green water, rich in microscopic algae. To produce it, leave a second tank of water in the sun with a pinch of fertilizer or a few lettuce leaves, and regularly transfer some of it. Alternatively, a knife-tip of diluted baker's yeast or powdered spirulina every two or three days is enough for a 100-litre tank. The number one mistake is overfeeding: the water becomes cloudy, oxygen levels drop and the culture collapses within a few days. Feed only when the water becomes clear again.

Aeration

Very gentle aeration, with a few large bubbles, maintains oxygen levels without creating a current. Fine microbubbles are not recommended: they can become lodged under the shell and bring the daphnia up to the surface. A few submerged plants, such as hornwort, provide oxygen and shelter.

Harvesting without exhausting the culture

1. Wait until the population is dense: the water teems with jumping specks.
2. Harvest at most one quarter to one third of the population per week, using a fine-mesh net.
3. Rinse the daphnia in clean water before feeding them to the fish.
4. Feed small quantities: uneaten daphnia survive in the pond without polluting it.
5. Replace some of the water in the tank each week and siphon off the deposits from the bottom.

And in the pond?

In a pond without fish or with few fish, daphnia often establish themselves spontaneously and actively contribute to water clarity. In a pond populated with koi or goldfish, they are quickly eaten: this is why a separate culture tank is essential if you want regular production. Introducing them into a very green pond can help clear it temporarily, provided the fish do not immediately feast on them.

Common problems

• The culture collapses: overfeeding, excessive heat, or a lack of oxygen.
• The daphnia turn red: the water lacks oxygen, so they produce hemoglobin. Aerate and feed less.
• Appearance of dark eggs: stressful conditions, overcrowding, or the end of the season. Harvest more frequently.
• Invasion by other organisms: cyclops, mosquito larvae. Cover the tank with mosquito netting and harvest selectively.

Read also

Natural shelters for zooplankton: Which floating plants are suitable for ponds and garden ponds?

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Frequently asked questions

— What is the difference between Daphnia magna and pulex?

Magna is larger and more productive; pulex is smaller and better suited to fry.

— What do daphnia eat?

Microscopic algae, bacteria, and fine particles. In culture: green water, or very small amounts of yeast or spirulina.

— Can daphnia clear up a green pond?

Yes, they filter microscopic algae, but in a pond with fish, they are quickly eaten.

— Why is my daphnia culture dying?

Most often caused by overfeeding, excessive heat, or a lack of oxygen. Feed less, provide shade, and aerate gently.

In summary

• Magna for productivity, pulex for fry.
• A 50 to 200 L tank in partial shade, with water free of chlorine and soap.
• Green water as ideal food, never overfeed.
• Very gentle aeration, with a few submerged plants.
• Harvest one-quarter to one-third of the population per week.

Last updated: September 28, 2026.