Medaka Egg Management and Hatching Guide | From Egg Collection to Fry Birth

Spawning Conditions and Egg Collection Timing

Breeding medaka is appealing because it is relatively easy to challenge even for beginners in fish-keeping. However, whether you know the egg management method or not greatly affects the hatching rate. To avoid wasting the eggs that are born, be sure to master the procedures from egg collection to hatching and initial fry care.

Medaka begin breeding behavior when conditions of water temperature of 20°C or higher and 13 hours or more of daylight are met. In outdoor cultivation, the spawning season is from late April to around September. With indoor cultivation, if you set up the environment with a heater and lighting, it is possible to spawn them throughout the year.

Females often spawn in the morning, and you can observe them swimming with bundles of eggs hanging from their bellies. Since eggs attach to aquatic plants or spawning substrates, collecting eggs daily is ideal. If you leave the eggs in the same container as the parent fish, there is a high risk they will be eaten, so move them to a separate container promptly.

Water lettuce roots and palm fiber have long been used for spawning substrates, but in recent years commercial sponge-type spawning substrates have become mainstream. Since egg collection is complete simply by moving the spawning substrate to a separate container, there is no need to handle the eggs directly by hand, making them easy to handle. Mop-type spawning substrates that imitate artificial algae are also commercially available, and when breeding multiple fish simultaneously, having a good number on hand prevents missing egg collections.

Egg Management | Water and Temperature Determine the Hatching Rate

Collected eggs are managed in a container separate from the parent fish. A small plastic case or container is sufficient.

Why Use Tap Water As-Is

For egg management, use tap water without chlorine removal as-is. The trace chlorine contained in tap water inhibits mold growth. The water level is fine as long as the eggs are fully submerged (about 5 cm deep). Maintain a clean environment by replacing all the water daily. When changing water, it is important to prepare water at a similar temperature so that temperature differences do not become too large.

Relationship Between Water Temperature and Hatching Days

The optimal temperature is 25–28°C. The guideline for medaka egg hatching is "accumulated temperature of 250°C-days"; hatching begins when water temperature × number of days reaches 250.

If water temperature is too low, hatching is greatly delayed, and the risk of mold increases in the meantime. Conversely, if high water temperatures above 30°C persist, the incidence of deformities is said to increase. Therefore, maintaining a stable temperature of 25–28°C through heater or room temperature management is the basis for improving the hatching rate.

How to Distinguish Fertilized and Unfertilized Eggs

It is also important to observe the eggs daily. Fertilized eggs are transparent, and as time passes, you can see black eyes and a body developing inside. On the other hand, white, cloudy eggs are unfertilized or molded eggs. Remove them with a dropper or tweezers as soon as you find them. If left alone, mold will spread to the surrounding healthy eggs.

Mold Prevention and Tips for Increasing Hatching Rate

The greatest enemy of eggs is water mold. Mold generated from unfertilized eggs spreads to neighboring fertilized eggs and prevents hatching. By combining the following three countermeasures, the hatching rate can be significantly improved.

① Early Removal of Unfertilized Eggs

Observe daily and promptly remove white, cloudy eggs. It is important to address the problem before one molded egg expands.

② Separate Eggs One by One

Freshly laid eggs are connected by thread-like filaments. By gently separating them with your fingertips to divide them one by one, you can prevent mold from spreading to adjacent eggs. Medaka eggs are more durable than they look, so they won't easily crush if pinched with your fingers. At first you may be hesitant to touch them, but once you get used to it, the work becomes much easier.

③ Use Methylene Blue

If you manage the eggs in water with methylene blue for ornamental fish diluted thinly (to the degree that it becomes a light blue color), you can effectively inhibit water mold formation. It is easy to obtain at home centers and aquarium shops, so by all means use it if you are serious about breeding. However, if the medication is too strong, it can put a strain on the fry immediately after hatching, so follow the prescribed dilution concentration.

Care of Fry (Newborn) Immediately After Hatching

Newborn fry are called "needle fish" and are about 4.5 mm long. How you raise them during this period greatly affects the subsequent survival rate.

No Feeding for the First 2–3 Days

Newborn fry have a yolk sac (nutritional sac) in their belly and can live on its nutrients alone. During this time, there is no need to feed, and in fact excess food causes water pollution, so refrain from feeding. Once the yolk sac is absorbed, feeding begins.

How to Choose Appropriate Food for Fry

Regular adult fish food is too large to fit in the mouth of newborn fry. Use powder-form fry-specific food, or crush adult food with your fingers and powder it before feeding. Feeding small amounts 3–4 times a day promotes efficient growth.

In addition, if you raise fry in green water (water with green coloration from plant plankton blooms), the fry have an environment where they can constantly eat fine food, improving survival rates. Paramecia culture and feeding is also effective, and many breeders practice it because it can dramatically increase fry survival rates. Using commercially available paramecia cultures or infusoria reduces the effort of culturing.

Precautions for Water Changes

While fry are small, there is a risk they will be sucked in during water changes. Avoid complete water changes and remove bottom waste a little at a time with a dropper. Once the fry are about 5 mm long, you can switch to water changes a little at a time.

Summary | Taking Your First Step in Breeding with BreederDirect

The key points to remember in medaka egg management are thorough egg collection, mold prevention with tap water, maintaining appropriate water temperature, and early removal of unfertilized eggs. During the fry stage, pay careful attention to feeding and water quality management, and if you can get through the first two weeks, the fry will quickly become hardy.

To succeed in medaka breeding, acquiring quality breeding stock is essential. BreederDirect allows you to purchase breeding pairs or trios (1 male, 2 females) directly from breeders with breeding experience. A major advantage is that you can easily consult with the breeders themselves about breeding tips and selection criteria for each variety.

From egg management to fry rearing, please experience the joys of medaka breeding with the support of experienced breeders.