Medaka/Rice Fish (Oryzias latipes): Comprehensive Protocol for Fry Rearing
- aquaterraobsession
- 5 days ago
- 10 min read
Feeding, Microfauna, Seasonal Cues, and Biofilm Augmentation
Introduction
Raising medaka fry (Japanese rice fish) is rewarding, though somewhat challenging. It combines elements of nutrition, water chemistry, seasonal biology, and microbial ecology.
Medaka fry, particularly during their so-called “hariko” stage—the first two weeks post-hatching—are extremely delicate and susceptible to mortalities primarily due to starvation and environmental instability. This guide provides a structured, research-backed protocol to maximize fry survival, optimize growth, and ensure robust health. It emphasizes detailed feeding schedules by developmental stage, careful integration of live foods and microfauna, the systematic use of Bacter AE to enhance biofilm and microbial richness, as well as environmental and seasonal cues such as temperature and photoperiod.
This protocol is grounded in both peer-reviewed science and robust aquaculture practices from Japanese breeders and the global hobbyist community.
Medaka Fry Developmental Stages
Understanding medaka developmental milestones is the prerequisite for fine-tuned fry management. Medaka hatch at approximately 4–5 mm in length, entering the critical “hariko” stage—the needle-like, newly hatched fry. This stage typically spans 0–14 days post hatching (dph) and is characterized by:
Extreme fragility and very small oral gape
Poor swimming and feeding ability
Reliance on yolk sac reserves for the first 2–3 days
Rapid transition to exogenous feeding by day 3–4
Medaka fry development can be subdivided as follows:
Stage | Days Post Hatch | Key Features | Critical Needs |
Yolk-sac fry (Hariko) | 0 – 3 | Absorbing yolk, no feeding required | Stable water, no exogenous food |
Early fry | 3 – 14 | Tiny mouth, weak swimmer | Frequent micro/nano-sized food |
Juvenile fry | 14 – 30 | Mouth enlarges, growth accelerates | Larger live/prepared foods, low stress |
Subadult/Young Fish | >30 | Fast growth, coloration appears | Standard fry foods/live foods |
Critical takeaway: Over 80% of medaka fry deaths occur during the hariko stage, nearly always due to starvation magnified by inappropriate food size or environmental instability.
Section 1. Feeding Schedules by Age and Developmental Stage
Successful medaka fry rearing necessitates frequent, appropriately sized, and nutritionally complete feedings. Feeding frequency and food type must match the developmental stage to avoid underfeeding, which is the principal cause of mortality.
Structured Feeding Schedule:
Days Post Hatch | Feeding Frequency | Primary Foods | Special Notes |
0–2 | — | None (rely on yolk sac) | No exogenous feed needed |
3–7 | 3–4 times daily | Infusoria, PSB, powdered commercial fry food | Ensure food is micro-sized; green water preferred |
7–14 | 3–5 times daily | Infusoria, Paramecium, micro-worms, powder | Integrate green water, start live foods |
14–30 | 2–4 times daily | Powdered fry food, baby brine shrimp, daphnia | Move to larger/tougher live foods as fry grow |
>30 | 2–3 times daily | Crushed pellet, brine shrimp, daphnia | “Wean” to subadult/adult diet |
Detailed Explanation and Analysis:
Days 0–2: Feeding is not required due to the presence of a yolk sac. Attempting to add food prematurely only risks fouling water.
Days 3–7: As the yolk sac is absorbed, fry must begin feeding. Their mouths are still extremely tiny, and food must be microscopic (infusoria, PSB, and ultra-fine powdered foods). Feeding at least three times daily is essential to avoid long fasting periods, as fry have minimal reserves and weak hunting ability.
Week 2 Onwards: Fry grow rapidly and can begin to ingest larger microfoods—Paramecium and microworms, along with specialized fry powders. Green water acts as both a food source and water stabilizer, providing phytoplankton and supporting microfauna populations.
Juvenile Stage (14–30 dph): Gradually transition to larger live foods (e.g., daphnia, baby brine shrimp) and less reliance on microscopic foods. At about 1 cm in size, fry can eat most prepared and live fry foods but should still be given frequent smaller meals.
Beyond 1 Month: At this stage, fry can be introduced to the adult foods suitably crushed if necessary, and the feeding frequency can be reduced in line with growth rates and seasonal activity.
Feeding Tips:
Never leave long intervals (>8 hours) without food in the first two weeks.
Avoid overfeeding—excess uneaten food quickly pollutes water and is a key driver of disease.
Target a “little and often” strategy—tiny amounts several times daily are preferable to single large feedings.
Section 2. Food Selections: Powdered Foods, Commercial Options, and Live Foods
The right food selections—by type, size, and quality—are pivotal in medaka fry rearing. This section details the critical food types and commercially available products, as well as best practices for home culturing and custom blending.
Powdered & Commercial Fry Foods
Key Properties of Ideal Medaka Fry Foods:
Particle size: Must be <100 microns for new hariko fry
High protein: 43%+ for fast growth and survival
Floating/suspension: Must not sink rapidly, as fry rarely forage at the substrate
Digestibility: Easily digestible, with probiotics and supplements when possible
Summary Table: Medaka Commercial Fry Food Options
Brand/Product | Key Features | Notes |
Comet Medaka Fry Food | Ultra-fine powder, garlic & probiotics, Bacillus natto, floats | Highly dispersible, supports gut health |
Hikari Medaka Baby (Kyorin) | Micro-powder, complete diet, slow-sinking, vitamin/mineral rich | Widely used in Japan, easy for fry to eat |
TetraMin Baby, Sera Micron | Generic powdered fry foods, used in absence of medaka-specific feed | Suitable but less tailored to medaka |
Analysis: These foods outperform regular crushed flakes in palatability, dispersibility, and nutrition. Use according to manufacturer instructions, always starting with small feedings and increasing as fry grow, but always minimizing waste to maintain water quality.
Live Food Integration
Infusoria
First live food to introduce—essential for fry that cannot accept even powdered food.
Provides movement (stimulates feeding response) and excellent nutrition.
Easily culturable at home using aquarium water, boiled lettuce, or yeast methods.
Java moss and green water tanks are natural infusoria sources.
Paramecium
Larger than infusoria, ideal as fry mouths grow slightly (Day 3 onwards).
Continuous availability in the rearing tank minimizes starvation risk.
Readily cultivable using boiled vegetables, rice rinsing water, or special cultures.
Daphnia and Rotifers
For older fry (>1 week) and juveniles: High protein and very active.
Live daphnia/rotifer cultures can be maintained in green water or aged tank water.
Daphnia also help clean the water by grazing on micro-algae.
Baby Brine Shrimp & Microworms
Introduced after 10–14 dph, once fry can physically ingest them.
Artemia nauplii are a gold-standard for protein and essential lipids.
Microworms are ideal if brine shrimp is unavailable, but should be introduced only when fry are large enough.
Green Water
Not a “food” in the conventional sense but an ecosystem resource.
Green water is phytoplankton-dense water, rich in microalgae, providing continuous nutrition and supporting microfauna (infusoria, rotifers, paramecia).
Benefits: constant food source, improved water quality via nitrogen uptake, and a “living buffer” against parameter swings.
Summary Table: Medaka Live Fry Food Options
Food Type | Pros | Use Case | Cons |
Infusoria | Essential for first feeding, continuous | Days 3–7+, micro fry | Needs home culturing, low bulk nutrition |
Paramecium | Perfect bridge food, easy to culture | Days 5–14, all fry | Can crash, needs starter culture |
Powdered Foods | Complete nutrition, easy to dose | Throughout fry stage | May pollute water if overused |
Green Water | Natural, 24/7 feeding, stabilizes tank | Hariko and early fry | Requires sunlight/growlight, balance needed |
Baby Brine Shrimp | Ultimate high-nutrition, moving prey | Juvenile fry >10 dph | Requires hatching setup, not for tiny fry |
Daphnia/Rotifers | Live, continuous growth, water cleaning | Juvenile, subadult | Needs live culture, size issue for small fry |
Section 3. Microfauna and Biofilm: Bacter AE Integration for Augmenting Growth and Survival
Role of Microfauna and Biofilm
While macro-sized live foods (daphnia, brine shrimp) are essential for advanced growth, microfauna and biofilm play a crucial survival role at the earliest stages. Hariko fry often rely on biofilm and tiny moving particles grazed continuously. A thriving micro-ecosystem in the rearing vessel multiplies grazing opportunities and moderates starvation risk.
Bacter AE: Microbial & Biofilm Booster
Bacter AE is a probiotic bacterial powder designed originally for shrimp, but highly effective in fry rearing contexts, including medaka. It provides:
Dormant Bacillus subtilis and Pediococcus acidilactici bacteria
Prebiotics: amino acids, polysaccharides, enzymes (xylanase, glucanase, protease, hemicellulase)
Biofilm stimulators: micro-particles colonized by beneficial microbes that fry graze upon
Mechanism & Best Practices:
On addition to the tank, Bacter AE seeds the water with dormant beneficial bacteria and the enzymes/prebiotics to thrive.
Within hours, biofilm begins forming on tank surfaces, plants, and substrate; free-floating particles also act as “edible dust”.
Continuous, gentle addition ensures a semi-constant supply of microbiofilm for fry, reducing starvation between formal feedings.
Dosage and Cautions:
Start with reduced rates: For fry tanks, use half or even quarter of the label dose until stability is confirmed (e.g., 1/4–1/2 measuring spoon per 10-20 liters every other day).
Avoid overdosing: Overapplication can result in oxygen depletion as bacteria multiply, or cause ammonia spikes and fouled water—especially dangerous in small or unfiltered fry containers.
Never add directly under substrate or without good tank oxygenation.
Monitor for cloudy water or surface scums, reduce dose if present.
Bacter AE—Benefits Analysis:
Greatly increases early fry grazing opportunities and survival rate.
Supports digestive health by supplementing amino acids and enzymes.
Outcompetes pathogens by fostering beneficial microbial dominance.
Summary Table: Bacter AE Integration
Benefit | Explanation |
Biofilm Growth | Provides a constant micro-food source for fry |
Improved Water Quality | Beneficial bacteria and enzymatic breakdown of waste |
Enhanced Survival | Reduces starvation risk by ensuring always-available nibbles |
Immunity Support | Probiotic bacteria stimulate immune functions |
Section 4. Green Water Protocols
Green water, also known as “ao-mizu,” is highly prized by Japanese and international medaka keepers as the “perfect fry food.” Raising fry in green water can raise survival rates from 20% to 70%+.
How to Make and Maintain Green Water
Basic Green Water Culture Steps:
Container: Use a white, clear, or light blue container to maximize light reflection.
Water Source: Start with aged aquarium water or water containing fish waste to provide nutrients for phytoplankton.
Fertilizer (Optional): For quick startup, add a minimal dose of liquid fertilizer (e.g., 2 mL per 10 L) or use “Hyponex” in regions where it’s available.
Light: Provide 6–12 hours daily of direct sunlight or, for indoor cultures, use full-spectrum LED grow lights.
Seeding: Add some starter green water, or a few cups from an aged tank, to accelerate phytoplankton development.
Aeration (Optional): Gentle aeration helps suspend phytoplankton and prevents stratification, but strong currents may disturb delicate fry.
Concentration Management:
The optimal color is a light green tea; too dark restricts oxygen, while too light means insufficient food.
Over-darkened water should be diluted or harvested for use in other fry containers.
Using Green Water for Fry
Add fry directly to established green water, or replace a portion of tank water with green water.
Combine with formal feedings (powdered food/infusoria) for maximal effect.
Monitor for oxygen depletion at night, particularly in small, dense containers; avoid over-thickened green water.
Tuning Green Water Through the Year:
Outdoor green water is easiest in spring/summer; in late fall/winter, shift to indoor cultures with strong artificial lighting as needed.
Maintain a “mother culture” tank for perpetual green water supply; seed new cultures regularly for ongoing fry rearing.
Section 5. Seasonal Cues, Photoperiod, and Environmental Synchronization
Medaka are seasonal breeders in the wild, responding powerfully to changes in temperature and photoperiod.
Photoperiod—Day Length Management
Natural Season: Medaka spawn and grow best when photoperiod exceeds 12 hours daily (typical of late spring to early autumn).
Artificial Regimes: To simulate optimal breeding/rearing season indoors, provide continuous 12–14 hour light cycles using full-spectrum LED aquarium lights.
Gradual changes in photoperiod can trigger breeding and accelerate fry growth; sudden drastic shifts are stressful.
Temperature Management
Optimal for Fry: 24–28°C (75–82°F). Fry grow fastest and are most robust with minimal daily swings—thermal stability trumps high temps.
Avoid strong daytime-nighttime swings, which can be fatal; >8°C variation in 24 hours strongly increases mortality.
Insulate using styrofoam boxes, use aquarium heaters with thermostats, or shade tanks to maintain desirable ranges.
Seasonal Synchronization
To rear fry out of season, artificially simulate “spring/summer” by setting photoperiod and temperature accordingly.
For overwintering fry or to lower metabolic demand, offer 8–10 hours of light and permit cooling to 18–20°C (64–68°F); feeding frequency can be safely reduced at lower temperatures.
Section 6. Container Design, Stocking Density, and Water Quality Management
Container Type, Size, and Layout
Material: White/light blue/clear plastic or glass favored for monitoring and light optimization.
Size: 2–5 liters ideal for home breeding batches; larger-scale operations use 10 L+ vessels or aquaria.
Stocking Density: No more than 20 fry per 2 L during first week; reduce to 10 fry per 2 L after reaching 1 cm size. Lower densities support faster growth and minimize cannibalism/interference.
Water Management
Aged, dechlorinated water is ideal for additions; never use raw tap for fry (except for hatching eggs, where chlorine is useful against fungus).
Water Changes: Limit to top-off for evaporation, especially in week 1–2. Frequent large water changes disturb biofilm, microfauna, and lead to survival drops.
Partial Changes: After 2 weeks, small partial changes (10–20% weekly) using identical-temperature, pre-treated water help maintain quality in growing tanks.
Filtration/Aeration: Avoid strong water currents or any filtration with suction—medaka fry are quickly exhausted and stressed by even gentle turbulence.
Sunlight and Lighting
Sunlight: Short exposures, especially to morning light, are strongly beneficial—stimulating growth, coloration, and immune function.
Too much direct sun, especially mid-day, risks overheating.
Artificial Light: LED aquarium lights (full spectrum, 12–14 hours) are highly effective substitutions for indoor-only rearing.
Section 7. Integrating Java Moss and Aquatic Plants
Java moss (and similar) provides surfaces for biofilm and infusoria colonization, shade from excessive light, and refugia for fry.
Aquatic plants help stabilize water chemistry, consume nitrogenous waste, and offer natural microfauna habitats.
Floating plants, such as dwarf water lettuce, frogbit, and hornwort provide refuge, biofilm grazing, and shade.
Section 8. Best Practices and Troubleshooting
Growth Monitoring and Health
Sort by size: Separate larger, fast-growing fry from runts weekly—this reduces cannibalism, allows prioritization of feed for smaller fry, and increases yield consistency.
Monitor for stunting: Overcrowding, poor nutrition, and large size disparities slow overall growth; aggressive thinning improves total health.
Inspect for illness: Vitamin B deficiency (from insufficient light) leads to immunodeficiency and poor growth; provide proper lighting and balanced nutrition.
Common Pitfalls
Overfeeding: Unconsumed food deteriorates rapidly in small volumes, driving ammonia spikes and disease outbreaks.
Overcrowding: Too great a density results in slow growth, cannibalism, and increased mortality.
Inadequate microfood: Failing to culture and maintain infusoria or paramecium is the main reason for high hariko mortality.
Stale green water: Over-concentrated or old green water may turn anaerobic, causing oxygen crises—monitor and dilute as required.
Advanced Troubleshooting Table
Issue/Observation | Likely Cause(s) | Remedies |
White fuzzy patches on eggs/fry | Fungal infection, poor water, no chlorine | Remove infected, use tap water for hatching eggs |
Fry disappear within days | Starvation, strong currents, predation | Provide denser microfood, remove adults, no current |
Smell or cloudiness in water | Overfeeding, biofilm overload | Reduce feeding, use Bacter AE less, partial water change |
Uneven growth/cannibalism | No size sorting, low microfauna | Sort by size, boost live microfood (paramecium, daphnia) |
Green water not developing | Not enough nutrients/light, predatory fauna | Add fertilizer, increase light, remove daphnia |
Conclusion
Raising medaka fry to robust juvenile and adult stages is a process deeply informed by their unique biology, environmental needs, and the pivotal roles of microfauna and biofilms.
With an appropriate feeding schedule (infusoria → paramecium → powdered/juvenile foods → live daphnia/brine shrimp), survival rates easily exceed 70–90%.
Microfauna integration—constant cultures of infusoria, paramecium, green water, and judicious use of Bacter AE—ensures fry always have food within reach, maximally reducing starvation risk and micronutrient gaps.
Seasonal cues (photoperiod, thermal regimes) are not only growth accelerants but the natural triggers for medaka breeding and best health; simulating spring/summer conditions year-round enables continuous successes.
Container design, water quality management, and plant integration round out a best practices approach: simple tanks, minimal disturbance, strong lighting, and weekly top-offs build the foundation for reliable, scale-appropriate results.
By following this comprehensive protocol—integrating the best of commercial diets, live foods, biofilm augmentation, and seasonally tuned environments—you not only ensure high fry survival and maximum growth, but foster a thriving, resilient medaka colony capable of weathering the inevitable challenges of aquarium life.
By leveraging these structured practices, medaka keepers and breeders can achieve professional-grade outcomes, with strong, colorful, and thriving ricefish fry developed in the home or laboratory environment.
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