Introduction to Medaka Breeding | Complete Guide from Spawning to Fry Cultivation

Medaka breeding is particularly accessible compared to goldfish or tropical fish, and with the right environment, you can observe spawning almost daily from spring through fall. The process from eggs hatching into tiny fry (called 'hariko') and growing into full-fledged adults is a major appeal unique to medaka keeping. This article provides a detailed step-by-step guide for those attempting to breed medaka for the first time, covering everything from the conditions that trigger spawning to egg management and fry rearing.

Three Conditions for Triggering Spawning

For medaka to spawn, three elements must align: water temperature, photoperiod, and nutritional status. If any one of these is lacking, the spawn count will decrease, so it's important to understand these fundamentals first.

Water Temperature

The optimal water temperature for spawning is 18–30°C, with the most active spawning occurring around 25–26°C. Below 18°C, spawning activity drops sharply, and it nearly ceases in winter. In outdoor tanks, the main breeding season is April to October, but in indoor setups, using a heater to maintain stable water temperature enables year-round breeding.

Photoperiod

Light exposure of 13 hours or more per day triggers spawning. In outdoor settings, you can rely on natural daylight cycles, but in indoor tanks, use a lighting timer to manage light intensity and duration. LED lights work well, but if the light is too weak, the effect diminishes; positioning the light near the top of the tank is recommended.

Nutritional Status

The nutritional status of parent fish directly affects spawn count and egg quality. Feed 2–3 times daily in amounts they can consume fully. Beyond standard pellet feed, offering high-protein, high-calorie spawning-specific feed and frozen bloodworms occasionally increases spawning output. In particular, ensuring good nutrition in early spring before the breeding season sets the stage for successful spawning.

Choosing Spawning Materials and Collecting Eggs

Types of Spawning Materials

Female medaka have a habit of laying eggs on aquatic plants and fibrous materials. Common spawning materials are as follows:

• Commercial spawning mats: Long-pile sponges and non-woven fabric types are most popular. Convenient and reusable.
• Aquatic plants (water lettuce, hornwort, watersprite, etc.): Closely mimics natural spawning environments and provides shelter for fry.
• Palm fiber: A traditional natural material long used for this purpose.
• DIY yarn spawning mats: Can be made by bundling yarn from a dollar store. Highly cost-effective.

Placing multiple spawning materials increases egg collection efficiency. Freshly laid eggs can be seen hanging from the female's abdomen before she deposits them on the spawning material.

Collecting and Transferring Eggs to Separate Containers

1. In the morning, remove the spawning material and check for attached eggs.
2. Gently rub the eggs off with your fingertip (fertilized eggs are elastic and can be handled with care without issue).
3. Transfer the eggs to a small plastic container or dollar-store storage box with water.
4. Return the spawning material to the parent fish tank.

Parent fish will eat eggs and fry, so separate rearing in a different container is essential. Also, if raising multiple strains, keep them in separate containers for each variety.

Egg Management Tips

• At 25°C, eggs hatch in approximately 10 days (lower temperatures require longer).
• Perform about 1/3 water changes daily to maintain cleanliness.
• Remove cloudy white eggs (unfertilized or dead) promptly before fungus spreads.
• Dechlorinated tap water has disinfectant properties as an alternative to methylene blue, and managing eggs with a small amount of chlorinated water is also effective.

Rearing Newly Hatched Fry (Hariko)

Newly hatched fry are called 'hariko' and are only about 0.5–1 mm in length. They still have a yolk sac in their abdomen, and for 2–3 days after hatching, they subsist on this nutrition, so feeding is not immediately necessary.

Appropriate Feeds for Hariko

During the hariko stage, the mouth is extremely small, so fine-particle foods are needed. Here are representative options:

• PSB (photosynthetic bacteria): A liquid-type food with extremely fine particles that hariko can consume. Also improves water quality.
• Paramecium: Live microorganisms with good digestibility and absorption that promote hariko growth. Self-culture is possible using commercial culture kits.
• Commercial powdered fry food: Fine particles and easy to handle. Brands like 'Medaka no Mai' and 'Tetra Min Baby' are well-known.
• Green water (algae-rich water): Culture water rich in plant plankton. An ideal environment where fry constantly consume trace amounts of food.

The basic rule is to feed 3–5 times daily in small amounts. Excessive uneaten food degrades water quality and causes mass fry death, so be careful not to overfeed.

Container Management by Growth Stage

Avoid cohabiting fry with adults until they reach 15–20 mm in length (about half the adult size). Cohabiting them earlier may cause stress from adults chasing them, or in the worst case, predation.

Keep Breeding Records for Thorough Strain Management

When raising multiple strains, you must record which parent each batch of fry came from. Simply applying masking tape to containers and noting the egg collection date, parent strain name, and hatch date makes future management significantly easier.

Additionally, if you aim for selective breeding, culling is important. Select superior individuals based on body shape, coloration, and fin morphology, and use them as parent fish for the next generation to approach your ideal traits. Building a record over time directly leads to improved breeding technique.

BreederDirect's Safety and Trust Mechanisms

If you're interested in selling medaka you've bred yourself, or purchasing quality medaka from a trusted breeder, BreederDirect is recommended. BreederDirect is a live-animal direct-sale platform connecting breeders and buyers directly with the following features:

• Direct breeder sales mean no middleman markup and fair-priced transactions.
• You can check sellers' rearing environments and management methods in advance, ensuring healthy individuals.
• Accumulated buyer reviews make it easy to find trustworthy breeders.
• Comprehensive support is in place to handle shipping issues.

Once you're comfortable with medaka breeding, please share your cultivated strains on BreederDirect. Many breeders are active on the platform as a place to share the joy of breeding with more people.