Hatching and feeding Artemia – a proven way to raise healthy koi fry

The successful rearing of koi larvae does not begin with the first feeding, but with the right timing. Many losses are not caused by poor-quality food, but by feeding too early or incorrectly. Newly hatched koi larvae initially have their own yolk sac, which fully nourishes them during the first days of life. Only once this yolk sac has been completely absorbed are they ready to take in external food.

Depending on the water temperature, this happens after about two to three days. In warmer water, the yolk sac is used up more quickly; in cooler water, it lasts longer. Before this point, koi larvae should not be fed, as they are neither able to actively ingest nor properly digest food. This is exactly where planning the Artemia hatching process becomes essential.

Artemia nauplii also need time to hatch themselves. Under optimal conditions—around 28 °C and the correct salinity—it takes about 24 hours for the nauplii to hatch. If you only start hatching Artemia once the koi larvae are already free-swimming, you are usually too late.

In practice, it has proven effective to start the Artemia hatching process on the first or second day after the koi have hatched. This ensures that fresh, nutrient-rich nauplii are available exactly when the larvae’s yolk sac has been depleted and they begin actively searching for food.

Hatching is carried out easily using PET bottles fitted with special caps that have two 4 mm connectors. One connector is used for continuous aeration, while the second—equipped with a valve—is later used for harvesting. The bottle is filled with water mixed with 30g of sea salt per liter and stirred thoroughly before adding the Artemia cysts.

Throughout the entire hatching period, strong aeration keeps the eggs suspended and evenly supplied with oxygen. At 28 °C, it takes about 24 hours for the first Artemia nauplii to hatch. The hatching time is influenced by water temperature: in cooler water it takes longer, while in warmer water it is faster. If multiple feedings per day are planned, several bottles are started at staggered times to ensure a constant supply of fresh nauplii.

After hatching, the aeration is switched off and the bottle is illuminated from below. Artemia react strongly to light and gather in the lower part of the bottle within a short time, while the empty eggshells float on the surface. Using the second connector with a valve, the nauplii can now be released in a controlled manner and collected in a fine sieve.

The image shows freshly harvested Artemia nauplii in a sieve. The uniform, intense orange coloration clearly indicates that these are vital, freshly hatched nauplii—exactly the quality that is ideal for koi larvae. The eggshells are not present in the sieve and should never be fed along with the nauplii, as they are indigestible and can negatively affect water quality.

A crucial step before feeding is thoroughly rinsing the Artemia. Since they hatch in highly saline water, they must be rinsed in the sieve using water from the koi rearing tank. This removes excess salt and prevents the sensitive koi larvae from being exposed to osmotic stress.

Especially during the first days of life, koi react extremely sensitively to sudden changes in water parameters. Properly rinsed Artemia are therefore not only healthier, but also far better tolerated.

As soon as the yolk sac has been completely absorbed and the larvae are actively free-swimming, the actual feeding phase begins. Freshly hatched Artemia nauplii are the ideal first food at this stage. Their movement in the water triggers the hunting instinct of the koi larvae and ensures a high feeding response.

Several small feedings spread throughout the day have proven effective, adjusted to stocking density and water quality. It is important to feed only as much as is consumed within a short time to avoid leftover food and unnecessary strain on water quality.

Successful koi rearing begins with patience and good planning. Feeding should only start once the larvae’s own yolk sac has been fully absorbed. Since Artemia themselves require time to hatch, starting them early and timing the process correctly is crucial. Freshly harvested, well-rinsed Artemia nauplii—like those shown in the sieve in the image—are the key to a healthy start for koi larvae.