As an avid aquarium enthusiast, I've always been fascinated by the captivating world of medaka rice fish. These delicate and vibrant creatures have captured the hearts of hobbyists worldwide, and for good reason. Their unique beauty, hardy nature, and fascinating breeding behaviors make them a true delight to keep and observe.
If you're a medaka enthusiast like myself, you're likely interested in not only keeping, but also breeding medaka. It's an enjoyable process that requires patience, attention to detail, and an understanding of the fish's natural breeding habits. That's why I'm excited to share with you my comprehensive guide on mastering the art of hatching medaka eggs.
Understanding Medaka Breeding Behavior
Before we dive into the hatching process, it's important to have a solid grasp of how medaka fish breed in their natural environment. These small, vibrant fish are egg-layers, but medaka are unique in that females carry fertilized eggs externally before depositing them.
In the wild, medaka typically spawn in shallow, slow-moving waters with plenty of aquatic vegetation. The females will attach their eggs to the underside of leaves, stems, algae, or other submerged surfaces. This protects the eggs from predators and provides them with the necessary oxygen and nutrients to develop.
In the aquarium setting, you can replicate these natural conditions by providing your medaka with a well-planted tank, gentle water flow, and suitable spawning substrates or better yet utilize a spawning mop. This will encourage the fish to engage in their natural breeding behaviors and increase the chances of successful egg-laying and hatching.
Preparing for Medaka Egg Hatching
Once you've set up the perfect breeding environment for your medaka, it's time to focus on the hatching process. Here are the key steps to ensure a successful outcome.
Egg Collection and Incubation
Carefully monitor your medaka tank for signs of spawning activity. When you notice the females holding eggs, gently remove the substrate or plants with the attached eggs or better yet utilize a spawning mop which the medaka will use for depositing eggs and make them easier to collect. While you may expect the eggs to be delicate and can easily damaged, this is simply not the case for fertilized viable eggs. They are firm and difficult to crush between your fingers. We recommend collecting the eggs with your fingers and even gently rolling the eggs between your finger tips to clean them. Of course another option to collecting individual eggs is to move the entire spawning mop to a hatching container.
While you may hear recommendations to place the collected eggs in a separate hatching container filled with dechlorinated, aged water, we have found eggs do well in fresh tap as well as aged water. The chlorine helps mitigate bacterial and fungal growth and will dissipate by the time the eggs hatch so won't harm the fry. Maintain the water temperature between 75-82°F (24-28°C), as this range is optimal for medaka egg development. As long as temps don't drop below 50°F for extended periods the eggs should be ok and continue to develop.
Warmer temps are preferable, but medaka eggs can handle a variety of temps. On the upper range fungal and bacterial growth can be problematic so we recommend 75-82°F as ideal but below 50°F and above100°F some eggs will likely still hatch. Many recommend gentle aeration and performing partial water changes every few days, but I do not provide aeration nor do water changes during the incubation phase. Keep in mind in the wild Medaka, or Japanese Rice Fish (Oryzias latipes), are naturally found in various shallow, slow-moving, or stagnant freshwater environments across Japan, China, Taiwan, and Korea.
Degree-Day Calculation Method
Where:
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Temperature = average daily water temperature (°C)
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Threshold = base temperature below which development stops. For medaka, it's around 10°C or 50°F
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Degree-Days required for hatching ≈ 252–288 (270 avg used for the calcualtion) DD
Example Calculation 1:
Average Water temperature = 78.8°F (26°C)
Formula (Fahrenheit version):
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Threshold temp (development stops) = ~50°F (equivalent to 10°C)
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Required DD for hatching = about 252–288 (270 avg) DD°F
DD/day = 78.8 - 50 = 28.8 DD/day
Days to hatch = 270 DD ÷ 28.8 DD/day ≈ 9.4 days
Example Calculation 2:
Average Water temperature = 68°F (20°C)
Example 2: Water temperature = 68°F (20°C)
DD/day = 68 - 50 = 18 DD/day
Days to hatch = 270 DD ÷ 18 DD/day ≈ 15 days
Summary Table (Fahrenheit version)
| Temp (°F) | DD/day | Days to Hatch |
|---|---|---|
| 64.4°F | 14.4 | ~15 days |
| 68°F | 18 | ~13.5 days |
| 75.2°F | 25.2 | ~11.3 days |
| 78.8°F | 28.8 | ~10.4 days |
| 82.4°F | 32.4 | ~9.6 days |
OK, that got a bit technical. We created a simple calculator for you below. Just input your average daily temperature, and it will estimate the days to hatch. You can calculate it for any temperature using:
BARCK Aquatics
Medaka Egg Hatch Time Calculator
*Based on 270 degree-days (DD). Actual hatch time varies with strain, oxygen, and water quality.
Monitoring Egg Development
Over the next 7-10 days, closely observe the eggs for signs of development. Healthy, viable eggs will gradually become more transparent, and you may even be able to see the embryo forming inside. Regularly check for any signs of fungal or bacterial growth, and promptly remove or separate any affected eggs to prevent the spread of infection.
Hatching and Fry Care
As the incubation period nears its end, you'll start to see the eggs begin to hatch. The fry will emerge as tiny, translucent creatures, often with a visible yolk sac. Newly hatched medaka are referred to as needles or paper machete by Japanese breeders. The fry will utilize their yolk sack for the first few days after hatching before being ready to eat. At this stage, it's crucial to provide the newly hatched fry with a nutrient-rich diet, such as powdered fish food or infusoria. A great way to encourage infusoria and provide some additional vitamins to your medaka is to use Photosynthetic Bacteria (PSB). See my blog about PSB by clicking the link below.
PSB Blog
Buy PSB here
We like to take regular adult medaka food and crush it between our fingers into a powder for the fry. We recommend this method and this medaka food-https://amzn.to/4opuoDe
Medaka fry benefit from having food available all the time. Small light dustings of food several times a day are recommended, but if kept outside, there will be some natural food sources created. Also, PSB encourages infusoria, which is available to the fry without feeding. Future blogs will discuss feeding in more depth.
We don't typically perform water changes for our fry containers, but we do move them to larger containers to grow out once they are a few weeks old. Also once a few weeks old we supplement their diet several times a week with live baby brine shrimp to jumpstart their growth, but this isn't required.
Troubleshooting Common Hatching Challenges
While the medaka egg hatching process can be incredibly rewarding, it's not without its challenges. Here are some common issues you may encounter and how to address them:
Fungal or Bacterial Infections
Fungal and bacterial infections are the bane of any aquarium hobbyist, and medaka eggs are susceptible to this. More commonly, the eggs were not fertile to begin with, which is why they were overcome by bacteria or fungus, but viable eggs can be affected by bacteria and fungus. If you notice any white, fuzzy growth or discoloration on the eggs, act quickly to remove the affected ones. We recommend gently rolling them between your fingers to remove the growth. If the eggs are soft, they are no longer viable and wouldn't have hatched anyway, so handling them is okay.
When hatching medaka (Oryzias latipes) eggs, controlling environmental factors like light exposure is important—especially in relation to fungus and bacterial growth, which can kill or damage the developing embryos.
Here’s a breakdown of how sunlight (or UV light exposure) affects fungus and bacteria during medaka egg incubation:
🌞 Effect of Sunlight on Fungus and Bacteria in Medaka Egg Hatching
1. UV Light Kills Pathogens
Ultraviolet (UV) rays, particularly UV-B and UV-C, can damage the DNA of bacteria and fungi, leading to reduced growth or death.
Exposure to natural sunlight, which contains UV-A and some UV-B, may help inhibit the growth of Saprolegnia spp. (a common egg fungus) and harmful bacteria on the egg surface.
2. Fungus Control via Sunlight
Sunlight may reduce fungal outbreaks by:
Raising water temperature slightly, improves embryo resistance.
Drying out surfaces where fungal spores might settle (in non-aquatic conditions).
Reducing spore viability in water and on equipment.
3. Bacterial Impact
UV-sensitive bacteria like Aeromonas or Pseudomonas (common in aquaculture) may be suppressed under regular sunlight.
However, biofilm-forming bacteria or those in shaded areas may still survive.
4. Risks of Excessive Sunlight
Overheating: Direct sunlight can overheat small volumes of water, which might kill embryos.
Algal blooms: Sunlight can stimulate algal growth, which may consume oxygen at night or promote bacterial growth.
Photodegradation: Light-sensitive compounds (like methylene blue used for fungal control) may break down.
🔬 Best Practices for Medaka Egg Hatching & Light Exposure
Factor Recommendation Sunlight Indirect or filtered sunlight is ideal. Prevent overheating and UV overexposure. Temperature Maintain around 26–28°C (78–82°F) for optimal development. Water cleanliness Change water frequently or use a gentle filtration system. (optional unless using a very small hatching container) Antifungal treatment Use methylene blue or NaCl (salt) at safe concentrations. (optional and usually not necessary, but if used be sure to move fry out of methylene blue treated water ASAP as it can impact fry survival). We ship eggs in methylene blue, but recommend non treated water in your hatching container.
📚 Supporting Research
Some studies suggest that:
Moderate UV exposure reduces microbial load on fish eggs.
Dark conditions may encourage fungal colonization due to stable, moist, and cooler environments.
For detailed research, look into:
Oryzias latipes egg development under different light regimes.
Aquaculture studies on UV sterilization and fungal prevention.
Conclusion
Mastering the art of hatching medaka eggs is a rewarding and fulfilling journey for any aquarium enthusiast. By understanding the fish's natural breeding behaviors, properly preparing the hatching environment, and troubleshooting common challenges, you can increase your chances of successfully raising a thriving medaka colony. We hope you enjoy the process from egg to adult and have many generations of medaka in your future.
At BARCK Aquatics, we're dedicated to providing our customers with the highest-quality medaka and shrimp, as well as the resources and knowledge to help them succeed in their aquarium endeavors. Whether you're a seasoned medaka breeder or just starting, we're here to support you every step of the way.
We offer a variety of medaka types as eggs. It's an economical way to obtain some unique strains. They are shipped in methylene blue to mitigate any bacterial or fungal issues.
Once your eggs arrive, place them in your hatching container and patiently wait for them to hatch. Our biggest piece of advice, don't try too hard. Viable medaka eggs are tough and can handle a variety of water parameters and conditions.
Be sure to only purchase from reputable sources. Good genetics are the key to healthy stock. At BARCK Aquatics, we not only started with quality stock, but we actively manage our stock for health, hardiness, and beauty. All our medaka are kept outdoors year-round, so the weak genes are weeded out. In addition, we only breed the highest quality individuals from each line to improve each generation. Most of our stock has been USA bred by us for over 4 years and some as long as 6 years. Support a US-based breeder and purchase your medaka from BARCK Aquatics.
So, what are you waiting for? Dive into the captivating world of medaka breeding and let the journey begin!
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