The Ultimate Guide to Raising Medaka Fry (Hariko): Preventing Common Deaths and Maximizing Survival
- medaka.uk
- Jul 10
- 5 min read
If you're new to breeding Japanese rice fish (M
edaka), you've probably experienced the heartbreak of watching your tiny fry disappear one by one. Don't worry – you're not alone! Raising medaka fry, especially during the critical "hariko" stage (the first 2 weeks after hatching), is considered the most challenging aspect of medaka keeping. But with the right knowledge and techniques, you can dramatically improve your survival rates from a dismal 20% to over 70%.
Understanding Medaka Fry: The Needle-Like Babies
Newly hatched medaka fry are called "hariko" (針子) in Japanese, which literally means "needle child" due to their incredibly thin, needle-like appearance. These tiny creatures are only about 4-5mm long and are extremely delicate during their first two weeks of life.
The #1 Killer: Starvation (Over 80% of Deaths!)
Here's a shocking truth that many beginners don't realize: the primary cause of hariko death is starvation, accounting for over 80% of mortality. Even when you think you're feeding them properly, they might actually be starving.
Why Do Hariko Starve So Easily?
Tiny mouths: Hariko have such small mouths that they cannot eat the same size food as adult medaka.
Poor swimming ability: They lack the swimming strength to chase after food or dive to the bottom for sunken particles.
Feeding frequency matters more than quantity: For hariko, feeding frequency is more important than the amount of food given.
The Solution: Proper Feeding Strategy
First 2-3 days: Do not feed newly hatched fry for 2-3 days as they survive on their yolk sac nutrients.
After day 3: Begin feeding with:
Powdered fry food: Crush regular fish food into a fine powder
Infusoria: Microscopic organisms that naturally occur in aged water
PSB (Photosynthetic Bacteria): Contains rich amino acids and vitamins that serve as nutrition for hariko growth.
Paramecium (Zourimushi): Live food that hariko can eat whenever hungry, significantly reducing starvation risk.
Feeding frequency: Feed 3-4 times daily with small amounts rather than one large feeding.
The Magic of Green Water
Green water contains large amounts of phytoplankton, resulting in high survival rates and fast growth for fry and juveniles. This is perhaps the single most effective method for raising healthy hariko.
How to Make Green Water
The process is surprisingly simple:
Use a light-colored container (white, clear, or light blue works best)
Add aged aquarium water (contains nutrients from fish waste)
Place in sunlight for about a week
Optional: Add a small amount of liquid fertilizer (2ml per 10L) to speed up the process
If you start with green water from the egg stage, you don't even need to move the fry to a different container.
Benefits of Green Water
Provides constant food supply (phytoplankton)
Acts as a natural water purifier by consuming harmful nitrogen compounds
Reduces the need for frequent feeding
Creates a more stable environment
Container Requirements
A 3-liter plastic container is recommended for household use as it's easy to clean, portable, and allows for good observation.
Water volume guidelines:
First week: 20 fry per 2 liters
After 1cm size: 10 fry per 2 liters
The Aeration Debate: To Bubble or Not to Bubble?
This is a crucial point that many beginners get wrong. Medaka have a habit of swimming against water currents, so using aeration can cause weak fry to become exhausted.
The verdict: Aeration is generally NOT recommended for hariko as the water current can be harmful.
If you absolutely must use aeration:
Reduce airflow to the absolute minimum by tying knots in the air tube
Use it only to prevent oil film formation
Ensure there are calm areas where fry can rest
Oxygen Alternatives
Instead of aeration, ensure adequate oxygen through:
Using containers with wide surface areas
Adding aquatic plants
Maintaining appropriate stocking density
Regular partial water changes
Temperature: The Goldilocks Zone
Hariko grow faster at higher temperatures, making indoor heating beneficial except in summer. However, temperature stability is more important than the exact number.
Ideal range: 24-28°C (75-82°F)
Critical point: Daily temperature swings (such as 30°C during the day dropping to 10°C at night) are extremely stressful and often fatal for hariko.
Solutions:
Use a heater for stable temperatures
Styrofoam containers provide excellent insulation
Avoid direct sunlight that can cause rapid temperature spikes
Water Quality: Less is More
One of the most counterintuitive aspects of hariko care is water management. Instead of changing water in fry tanks, simply add water to replace what has evaporated. Water changes will lower survival rates.
Key points:
Use aged, dechlorinated water for additions
Interestingly, eggs can be kept in tap water (with chlorine) as it helps prevent fungal growth
Maintain stable conditions above all else
The Importance of Sunlight
Exposure to morning sunlight makes fry stronger, grow faster, and increases survival rates. This is one of the most underappreciated factors in fry care.
Medaka lacking Vitamin B (from insufficient light) become susceptible to illness with weakened resistance.
If natural sunlight isn't available:
Use full-spectrum LED lights
Provide 12-14 hours of light daily
Position containers near windows when possible
Common Mistakes to Avoid
Overfeeding: Excess food quickly pollutes the small water volume
Using dark containers: Makes it hard to monitor fry and spot problems
Keeping with adults: Adult medaka will eat hariko if kept in the same container
Adding strong water flow: Exhausts the tiny fry
Frequent water changes: Disrupts the delicate ecosystem
Timeline for Success
Days 1-3: No feeding, yolk sac nutrition only
Days 4-14: Critical period, feed frequently with appropriate foods
Week 2-4: The most difficult time is when juveniles have grown slightly larger than fry, as their mouths are still small but energy needs are high
After 1cm size: Can return to parent tank safely
Creating Your Success Setup
Here's a simple recipe for maximum survival:
Container: 3-5 liter clear or white plastic container
Water: Green water or aged aquarium water
Food: PSB + powdered fry food + paramecium culture
Location: Bright area with morning sun
Temperature: Stable 25-27°C
Maintenance: Daily feeding 3-4 times, weekly water top-offs
Advanced Tips for Higher Survival Rates
Start green water before eggs hatch: It's recommended to make green water before collecting eggs.
Use multiple containers: Separate fry by size to prevent cannibalism
Add Java moss: Provides infusoria and hiding spots
Monitor growth differences: Move fry that grow about twice as large as others to prevent cannibalism
Conclusion
Raising medaka hariko successfully is all about understanding their unique needs. By focusing on preventing starvation through frequent feeding and green water, maintaining stable conditions, and avoiding common pitfalls like strong aeration, you can achieve survival rates of 70% or higher.
Remember, if you can meet the right conditions – appropriate water temperature, water quality, feeding frequency, stocking density, and overall suitable environment – you can generally achieve survival rates above 90%.
The key is patience and consistency. Every medaka keeper loses fry when starting out, but with these techniques, you'll soon be raising healthy schools of these beautiful fish. Happy medaka keeping!