Explains medaka egg management methods. Introduces egg collection, distinguishing unfertilized eggs, water mold prevention strategies, and hatching periods.
Key Takeaways
Explains medaka egg management methods. Introduces egg collection, distinguishing unfertilized eggs, water mold prevention strategies, and hatching periods.
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While medaka are known as relatively easy-to-breed fish, improper egg management can lead to water mold growth and reduced hatch rates, resulting in wasted life. Understanding the complete process—from egg collection on spawning mats, selecting fertilized eggs, setting up the hatching environment, to caring for fry after hatching—is the key to successfully raising healthy fry. Whether you're attempting to breed popular varieties such as Yangguifei medaka or Midori medaka, be sure to first master the basics of egg management. This article explains each step in detail so that even beginners can start implementing these practices right away.
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Medaka have a habit of spawning in the early morning, so daily observation is essential. If you're using sponge or palmetto spawning mats, check them carefully for attached eggs.
Medaka eggs are approximately 1 to 1.5 mm in diameter and are elastic enough to withstand being touched without breaking. Many beginners worry that eggs will burst if touched, but fertilized eggs can generally withstand light pressure from your finger without issue. It's important to work with confidence.
Immediately after spawning, females may have a mass of eggs hanging from their abdomen. Collecting them at this timing increases the hatch rate.
When removing eggs, work quickly to minimize stress to the female. Avoid exposing them to air for extended periods.
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If you manage all collected eggs without selection, water mold may develop on unfertilized eggs and spread to nearby fertilized eggs. It's important to always perform selection.
| Type | Appearance | Feel |
|---|---|---|
| Fertilized eggs | Transparent to pale yellow, interior is clear | Elastic, does not burst when pressed with finger |
| Unfertilized eggs | Milky white, opaque | Soft, easily crushed with light pressure |
As time passes, fertilized eggs will develop visible black eyes (eye spots) inside. Once you can see these, hatching is just around the corner. Remove unfertilized eggs immediately upon discovery. Leaving them in place creates a breeding ground for water mold and negatively affects nearby healthy eggs.
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Large aquariums are not necessary for egg management. Inexpensive plastic cups or containers from dollar stores are sufficient. If managing a small number of eggs, a single cup will do. However, if eggs are too densely packed, water mold spreads easily, so manage them with adequate spacing.
One of the biggest misconceptions in medaka egg management is that "water must be dechlorinated." In fact, the opposite is true—untreated tap water (not dechlorinated) is best. Chlorine has sterilizing properties that suppress water mold growth.
The "accumulated temperature" required for medaka eggs to hatch is approximately 250°C. This is expressed by the formula "water temperature × days = 250."
| Water Temperature | Approximate Days to Hatching |
|---|---|
| 20°C | Approximately 12–13 days |
| 25°C | Approximately 10 days |
| 28°C | Approximately 9 days |
| 30°C | Approximately 8 days |
During summer, water temperature naturally rises and hatching accelerates, but temperatures exceeding 30°C can damage eggs. Ideally, maintain water temperature around 25–28°C.
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As hatching approaches, you'll be able to see fry moving inside the egg with the naked eye. Once you can clearly see the black eye spots and the egg appears slightly larger overall, hatching is imminent. Observe closely from this point onward.
Newly hatched fry are extremely small (approximately 3–4 mm) and carry a yolk sac on their belly. They can survive for 2–3 days on the nutrition from the yolk sac alone, so feeding immediately after hatching is not necessary.
Fry lying on the bottom and not moving
→ Water temperature may be too low. Ensure it stays above 20°C.
Hatch rate is significantly low
→ Water mold or unfertilized eggs are often the cause. Be thorough with selection after collection and daily water changes.
Fry hatched but disappeared the next day
→ They were likely eaten by adult fish. Always isolate fry in a dedicated container.
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The key to successful medaka breeding is carefully executing the complete process of egg collection → selection → setting up the management environment → post-hatching care. Three points in particular are common stumbling blocks for beginners, yet are critically important: "daily water changes with untreated tap water," "immediate removal of unfertilized eggs," and "isolating fry from adult fish after hatching."
For those wanting to start with selecting breeding pairs and varieties, we recommend "Br-choku," where you can purchase directly from breeders. You can confidently obtain healthy parent fish and fertilized eggs carefully managed by reliable breeders, ensuring a solid start to your breeding efforts. Being able to ask questions directly to knowledgeable breeders about varieties and breeding practices is a major source of reassurance for beginners.
