Medaka Egg Management & Fry Care — Tips to Improve Hatch Rate

Breeding medaka is relatively straightforward, but egg management and fry rearing require specific techniques. Newly hatched fry (called "needle fry" or hariko) are extremely delicate, and many will be lost without proper care. This article provides a detailed guide from egg collection through to the stage where fry become stable.

Egg Collection Methods

Medaka begin spawning when water temperature exceeds 20°C and daylight hours reach 13 or more. Females produce eggs in the morning, carrying them attached to their abdomen for a while before depositing them on aquatic plants or spawning mops.

Types of spawning beds: Water hyacinth roots are the most preferred natural spawning bed. Artificial spawning mops (made from palm fiber or sponge) are convenient and easy to collect. Handmade mops from bundled yarn also work well.

Egg collection: Remove the entire spawning bed from the parent tank and transfer it to a separate container. You can also pick eggs off individually with your fingers. Medaka eggs are surprisingly firm and will not burst when gently pinched. If adhesive threads are present, gently roll them between your fingertips to separate the eggs.

Why separate from parents: Adult fish will eat their own eggs and hatched fry. Always manage eggs in a separate container.

Egg Management

Container: Shallow plastic containers or food storage containers are easy to work with. A water volume of 500 ml to 1 liter is sufficient.

Water: The key tip is to use untreated tap water directly. The chlorine has a sterilizing effect that prevents fungal growth (water mold) on the eggs. Tap water chlorine is harmless to eggs, but switch to dechlorinated water as hatching approaches.

Water temperature and hatching time: Hatching time can be estimated as "250 divided by water temperature (°C)." At 25°C, eggs hatch in about 10 days; at 30°C, in about 8 days. Higher temperatures speed hatching, but temperatures above 30°C increase the rate of deformities.

Daily management: Remove infertile eggs (those that turn white and cloudy) as soon as they are found. Leaving infertile eggs causes water mold to develop and spread to nearby fertile eggs. Fertile eggs are transparent, and eyes become visible inside after a few days. Change the water every 2-3 days to keep it clean.

Methylene blue: Adding a few drops of methylene blue to the water enhances mold prevention. A light blue tint is sufficient.

Managing Newly Hatched Needle Fry

Newly hatched fry are called "needle fry" (hariko), measuring only about 2-3 mm in length.

No feeding for the first 2 days: Needle fry have a yolk sac attached to their belly that provides nutrition for approximately 2 days after hatching. No feeding is necessary during this period.

First foods: After the yolk sac is absorbed, the first food should be powdered fry food (crushed to powder) or paramecium (infusoria). Paramecium can be cultured at home with a culture kit and is the perfect size as live food for needle fry. PSB (photosynthetic bacteria) is also popular as an initial food for needle fry.

Green water: The most convenient and effective feeding environment for needle fry is green water (water rich in phytoplankton). Needle fry can continuously feed on plankton in green water, dramatically reducing the risk of starvation. Exposing tank water to sunlight will naturally turn it into green water.

Stage-by-Stage Fry Care

Needle fry stage (up to 1 cm): Keep water changes to a minimum. Use a pipette to remove debris from the bottom, avoiding large water changes. No water flow -- do not use a filter. Air pumps should be very gentle or turned off entirely.

Fry stage (1-1.5 cm): Feeding baby brine shrimp promotes rapid growth. At this stage, fry can also eat fine-grained artificial fry food. Growth rates vary even among siblings hatched at the same time. Larger individuals may eat smaller ones, so separate by size if differences become noticeable.

Juvenile stage (1.5 cm+): Fry can now eat crushed adult food. Sponge filters can be used at this point. Once they reach about 2 cm, the risk of being eaten by adults is low enough to introduce them to the main tank.

Find Breeding Stock on BriChoku

If you want to enjoy breeding medaka, selecting good parent fish is the starting point. On BriChoku, you can purchase medaka with well-established variety traits directly from breeders. Breeders are available to offer advice on breeding and egg management, providing peace of mind even for first-time breeders.