Medaka Breeding Guide | From Spawning Conditions to Fry Rearing | ブリちょく
Medaka| ✍️ BreederDirect Editorial
Medaka Breeding Guide | From Spawning Conditions to Fry Rearing
A detailed explanation of the water temperature and light duration conditions necessary for medaka breeding, setting up spawning beds, egg collection and management, feeding methods for fry (needle fish), and management methods for each growth stage.
Key Takeaways
A detailed explanation of the water temperature and light duration conditions necessary for medaka breeding, setting up spawning beds, egg collection and management, feeding methods for fry (needle fish), and management methods for each growth stage.
Establishing Basic Conditions for Medaka Breeding
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The BreederDirect editorial team specializes in care information for animals, covering veterinary care and breeding to deliver beginner-friendly guides.
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Medaka are known as hardy fish that are easy to breed, but creating an appropriate environment is essential to encourage spawning. In nature, they enter their breeding season from spring to autumn, so reproducing those conditions in captivity is key. Refer to the Water Temperature Management Guide | By Season as well.
Water temperature is the most important factor, with 18°C or higher being the standard for spawning. The optimal water temperature is 25–28°C, and maintaining this range consistently increases the spawning rate. While heating allows breeding even in winter, natural breeding from spring to autumn is ideal when considering the medaka's stamina.
Light duration is also a crucial factor for triggering spawning. A light period of 13 hours or more per day is necessary, and in indoor rearing, a lighting timer should be used to ensure a stable photoperiod. When kept near a window, seasonal changes in sunlight create a natural spawning rhythm.
As for individual maturity, the condition is that they be adult fish that are at least 2–3 months past hatching. Once they reach approximately 2 cm in body length, most individuals are in a state capable of breeding. Additionally, providing nutrient-rich live foods such as bloodworms and daphnia before spawning improves both the number of eggs and their quality. The Feeding Guide | Adults also explains food management during the breeding season.
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Observing Spawning and Egg Collection Methods
During the spawning period, females swim each morning with a mass of eggs hanging from their belly. Once this is confirmed, wait for the eggs to attach to the spawning bed. Spawning primarily occurs from early morning through late morning.
Choosing the spawning bed is important. The roots of floating plants such as water sprite are excellent as spawning beds and create a natural-like environment. However, depending on the season and indoor conditions, they can wither easily, so commercially available artificial spawning beds (palm fiber or dedicated mat-type) are also very effective. The Egg Collection Techniques | Spawning Beds explains the placement and management of spawning beds in detail.
Collecting eggs once a day is the basic rule. When collecting eggs from the spawning bed, gently pinch them with the tip of your finger or extract them with a thin dropper. If eggs are stuck together, do not forcibly separate them; move them as a clump.
The condition of eggs can be determined by appearance. Clear to light yellow eggs are fertilized and a sign of health. White, turbid eggs are infertile or dead, and if left alone, mold will spread to other eggs, so remove them as soon as you find them.
The most important is separation from the parent fish. Medaka have a habit of actively eating their own eggs and fry. Moving the spawning bed to a separate container or removing the eggs and managing them in a dedicated hatching container is the key to breeding success. Be sure to check the Complete Egg Management Guide | Egg Collection as well.
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Carefully Managing Egg Hatching
The number of days for eggs to hatch depends on water temperature. At 25°C, it takes approximately 10–14 days, with "accumulated temperature of 250°C" as a guideline. For example, at 20°C it takes about 12–13 days, and at 30°C about 8 days.
The hatching container can be a small plastic container, a food storage container, or the bottom half of a plastic bottle. Use water from the original tank or dechlorinated tap water. Since the chlorine in tap water also has a mold-inhibiting effect, using a small amount of chlorinated tap water is also an option.
Adding methylene blue at about half the specified concentration is effective for preventing mold on eggs. However, it is too strong a stimulus for fry after hatching, so gradually perform water changes to dilute it once hatching begins. The Hatching Guide | Egg Collection also introduces 2026 hatching management techniques.
Water flow should be gentle enough to circulate with only light aeration. Water flow that is too strong damages eggs and fry after hatching. Use an air stone and adjust it so that only imperceptibly fine bubbles emerge. Remember to perform daily water changes of about 10–20% to prevent water quality deterioration.
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Raising Fry (Medaka Hatchlings)
Fry immediately after hatching are called "needle fish" and are about 2–3 mm in body length. This is the most delicate stage in breeding. For 2–3 days after hatching, they live on the nutrition from the yolk sac attached to their belly, so it is fine not to feed them during this time.
The most effective initial food is green water (algae water). It is rich in phytoplankton and creates an environment where hatchlings can constantly eat at their own pace. If it is difficult to prepare green water, use commercially available powdered fry food in very small amounts, divided into about 5 feedings per day. Feeding too much at once deteriorates water quality, so it is important to feed "only the amount with no leftovers."
PSB (photosynthetic bacteria) and infusoria (microorganisms) are initial foods that are excellent for fry digestion and absorption. Infusoria in particular occur naturally in tanks, but intentionally culturing and feeding them achieves a more stable survival rate. The Complete Fry Rearing Guide | Hatching teaches consistent management methods from hatchlings to juveniles.
Water changes should be done carefully with a dropper. If the suction is too strong, the fry will be sucked up too, so slowly extract only the waste that has accumulated at the bottom. Keep water changes to about 10–15% of the total volume, and ideally perform them in small amounts daily.
Always separate and manage fry of different sizes. Within 1–2 weeks, individual differences become clear. To prevent "cannibalism" where larger individuals eat smaller ones, move individuals that differ in body length by more than 2 times to separate containers.
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Recording Growth and Steps to the Next Breeding
Fry growth varies depending on water temperature and feed quality, but as a rough guide:
1 week after hatching: About 5 mm body length at 25°C. Still called the needle fish stage
2–3 weeks after hatching: 7–8 mm. Consider transitioning from powder food to fine granular food
1 month after hatching: 10–15 mm. They become somewhat stronger and management becomes easier
2–3 months after hatching: 15–20 mm, nearly adult size. Some individuals become capable of breeding
Before returning them to the original tank as adults, check their size and body shape/color. If working on medaka breeding, confirming how well the target traits are fixed can improve selection accuracy for the next generation.
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Summary | Key Points for Breeding Success and the Importance of Parent Selection
Medaka breeding is fundamentally about three steps: "establishing conditions, protecting eggs, and carefully raising fry." In particular, never neglecting to isolate eggs and fry from parent fish has a major impact on survival rate.
The enjoyment of breeding is not only in raising fry but also in how parent quality determines the next generation's characteristics. By breeding from individuals with good coloration and bloodlines with high fixation rates, more beautiful medaka are likely to be born.
Br-choku's medaka category features parent fish information submitted directly by breeders and individuals with clearly stated fixation rates. Since the environment is set up for selecting parent fish with the perspective of "wanting to breed from this individual," those who want to enjoy breeding can use it with confidence. Since you can purchase directly from established breeders, you can meet individuals with stable quality.
