Complete Guide to Raising Medaka Fry — Management from Hatching to One Month

Medaka Fry Rearing: The Most Challenging Aspect of Medaka Breeding

While medaka breeding is relatively straightforward, raising the hatched fry (fry stage, called "hashigo") to adult size is actually the most challenging step. The mortality rate at the fry stage is high, and many breeders struggle with the problem of "eggs are being laid but the fry simply won't grow."

Most fry failures can be attributed to three factors: "insufficient feeding," "water quality degradation," and "being eaten by parent fish or other fish." Proper management of these three points will allow you to successfully raise nearly all fry to healthy adults.

Egg Management and Preparation of Hatching Containers

Collect eggs daily from the water plants or spawning substrate where they were laid and transfer them to a dedicated hatching container. Leaving eggs in the parent fish's tank carries a high risk of them being eaten.

A small container of 100-500ml (plastic containers, plastic bottles, etc.) is sufficient for a hatching container. Using tap water (with chlorine) helps prevent fungal growth. At this stage, relying on the sterilizing effect of chlorine is the basic approach, rather than performing water changes.

At water temperatures of 25-28°C, the incubation period is approximately 10-14 days (higher temperatures result in earlier hatching). Adding a small amount of methylene blue (just enough to create a light blue tint) provides antifungal protection.

Post-Hatching Care (Fry Stage: Hatching Through 1 Week)

Immediately after hatching, fry are 2-3mm in length and extremely thin, like a needle (hence called "hashigo" or fry). At this stage, their mouths are very small and cannot consume standard medaka food.

Feeding: Provide PSB (photosynthetic bacteria), infusoria, or powdered fry food 3-5 times daily in small amounts. Rearing in green water (containing phytoplankton) provides a constant supply of microorganisms for the fry to eat, dramatically improving survival rates.

Water Changes: Care must be taken with water changes during the fry stage. Performing a complete water change in a small container risks shocking the fry with poor water quality. It is safer to simply remove debris from the bottom with a pipette and perform partial water changes of small amounts approximately once per week.