How to Make Green Water for Medaka: The Complete Guide to Creating Nature's Perfect Fry Food

Green water might look like something went wrong with your aquarium, but for medaka breeders, it's liquid gold. This nutrient-rich, phytoplankton-filled water can boost fry survival rates from 20% to over 70%, act as a 24/7 buffet for your fish, and create a more stable environment. But how do you actually make it? After years of trial and error, here's everything you need to know about creating and maintaining perfect green water for your medaka.

What is Green Water?

Green water (known as "ao-mizu" 青水 in Japanese) is water that has turned green due to a bloom of microscopic algae called phytoplankton. Think of it as a living soup of nutrients that your medaka can graze on throughout the day. It's particularly crucial for raising fry (baby medaka) who need to eat frequently but have mouths too small for regular food.

Why Green Water is a Game-Changer for Medaka

Before diving into the how-to, let's understand why green water is so valuable:

1. Constant Food Supply: Fry can eat whenever they're hungry, dramatically reducing starvation deaths

2. Water Quality Management: Phytoplankton consume harmful nitrogen compounds from fish waste

3. Natural Nutrition: Provides essential vitamins and minerals for healthy growth

4. Live Food Cultivation: Perfect for breeding daphnia and other microorganisms

5. Stable Environment: Acts as a natural buffer against water parameter swings

   
   

Watch This First: Visual Guide to Green Water

For a visual demonstration of green water techniques, check out this helpful video:

https://www.youtube.com/watch?v=r7y5Lxotmmk&t=306s

Method 1: The Natural Approach (2-4 Weeks)

This is the traditional method that requires patience but produces the most natural results.

What You Need:

• Light-colored container (white, clear, or light blue work best)

• Aged aquarium water (contains nutrients from fish waste)

• Sunny location

• 2-4 weeks of patience

Steps:

1. Fill your container with water from an established medaka tank

2. Place in direct sunlight for at least 6 hours daily

3. Wait and watch as the water gradually turns green

4. Once it reaches a light tea color, it's ready to use

Pro tip: The key is using water that already contains fish waste – this provides the nutrients phytoplankton need to bloom.

Method 2: The Hyponex Method (3-7 Days)

For those who need green water quickly or don't have established tank water, liquid fertilizer is your friend.

What You Need:

• Hyponex (liquid plant fertilizer)

• Dechlorinated water

• Light-colored container

• Sunny location or aquarium light

The Magic Formula:

• 10 liters of water : 2ml of Hyponex

Steps:

1. Fill container with dechlorinated water

2. Add Hyponex at the ratio above (use a syringe for accuracy)

3. Optional: Add a cup of established tank water as a "starter"

4. Place in bright light

5. Stir daily to prevent settling

6. Green water should develop in 3-7 days

WARNING: Never exceed the recommended dose! Too much fertilizer can create toxic conditions that will kill your medaka. When in doubt, use less.

Method 3: Live Chlorella Method (Instant to 3 Days)

This is the fastest and most reliable method, though it requires purchasing live chlorella culture.

What You Need:

• Live chlorella concentrate

• Dechlorinated water

• Any container

Steps:

1. Add 1cc of live chlorella per liter of water

2. Mix well

3. Can be used immediately or cultured for stronger concentration

4. Place in light to maintain and strengthen the culture

Note: Live chlorella must be refrigerated and typically lasts 2-4 weeks. Unlike powdered chlorella, live cultures won't sink to the bottom.

Indoor Green Water Production

No outdoor space? No problem! You can create green water indoors with these modifications:

Light Requirements:

• Use full-spectrum LED grow lights

• Minimum 10-12 hours of light daily

• Position lights 6-12 inches above water surface

• Higher wattage = faster green water development

Best Practices for Indoor Culture:

1. Use white or clear containers to maximize light penetration

2. Add gentle aeration to prevent stagnation

3. Maintain temperature between 20-30°C (68-86°F)

4. Consider using a timer for consistent light cycles

The Perfect Green Water Concentration

Not all green water is created equal. Here's how to achieve the optimal concentration:

Visual Guide:

• Too Light: Can barely see green tint - needs more time/nutrients

• Perfect: Light green tea color - can faintly see white object at 30cm depth

• Too Dark: Can't see anything below surface - dilute immediately

Why Concentration Matters:

• Too light = insufficient food for fry

• Too dark = oxygen depletion at night (dangerous!)

• Perfect = balanced nutrition without risks

Troubleshooting: Why Your Green Water Isn't Working

If your water stays crystal clear after weeks of trying, here's what might be wrong:

Common Problems and Solutions:

1. Insufficient Light

• Solution: Move to brighter location or add artificial lights

• Needs minimum 6 hours direct light daily

2. Lack of Nutrients

• Solution: Add more fish waste water or use fertilizer method

• Ensure no activated carbon in filters (removes nutrients)

3. Nutrient Competition

• Solution: Remove plants and substrate that absorb nutrients

• Use bare containers for green water production

4. Wrong Container Color

• Solution: Switch to white or clear containers

• Dark containers absorb light and inhibit growth

5. Presence of Daphnia or Other Grazers

• Solution: Remove all organisms that eat phytoplankton

• Culture green water separately from live foods

6. Water Too Cold

• Solution: Maintain temperature above 20°C (68°F)

• Warmer water (25-30°C) speeds up the process

Maintaining Your Green Water Culture

Once established, green water is easy to maintain:

Daily Care:

• Stir or aerate gently to prevent settling

• Monitor color concentration

• Check for any bad smells (indicates wrong type of algae)

Weekly Maintenance:

• Test pH (should be stable around 7-8)

• Add fresh water to replace evaporation

• Harvest some for use, replace with fresh water

Long-term Culture:

• Keep a "mother culture" always running

• Use harvested green water to start new batches

• Can maintain indefinitely with proper care

Using Green Water Safely

For Fry:

• Dilute to light green before adding fry

• Monitor oxygen levels, especially at night

• Combine with regular feeding for best results

For Adults:

• Can use stronger concentrations

• Excellent for conditioning breeding fish

• Reduces need for frequent feeding

Warning Signs to Watch For:

• Foul smell (indicates bad bacteria, not phytoplankton)

• Brown or black color (wrong type of algae)

• Fish gasping at surface (oxygen depletion)

• Sudden die-offs (toxic conditions)

Advanced Tips from Japanese Breeders

The "Seed Water" Method:

Once you have good green water, use it to jumpstart new batches:

1. Mix 1 part green water with 9 parts fresh water

2. Add nutrients (fish waste or fertilizer)

3. New batch ready in half the time

The "Perpetual Culture" System:

1. Set up multiple containers in rotation

2. Harvest from container 1 while container 2 develops

3. Always have green water available

4. Never completely empty a container

Seasonal Considerations:

• Spring/Summer: Easy production outdoors

• Fall: Start indoor cultures before it gets cold

• Winter: Rely on artificial lights and heating

• Year-round: Maintain at least one indoor culture

Alternative Products

If Hyponex isn't available in your area, these alternatives work well:

Commercial Options:

• Easy Green: Designed specifically for aquarium use (5ml per 5L)

• Miracle-Gro: Use at half the Hyponex concentration

• Seachem Flourish: Aquarium-safe but more expensive

DIY Nutrient Mix:

For the adventurous, you can create your own nutrient solution:

• 1 part aquarium plant fertilizer

• 1 part dissolved fish food

• 10 parts water

• Use sparingly and test carefully

Common Mistakes to Avoid

1. Overdosing Fertilizer: "More is better" doesn't apply here – excess nutrients can be toxic

2. Using Tap Water Without Dechlorinating: Chlorine kills phytoplankton

3. Keeping in Complete Darkness: Even indirect light is better than none

4. Adding Fish Too Soon: Let green water establish for at least 3 days first

5. Ignoring Temperature: Cold water significantly slows production

6. Using Black Containers: Dark colors absorb light and inhibit growth

The Science Behind Success

Understanding why green water works helps you troubleshoot problems:

Phytoplankton Needs:

• Light for photosynthesis

• Nitrogen from fish waste or fertilizer

• Phosphorus for growth

• Trace minerals

• Proper temperature range

• Water movement to stay suspended

The Nutrient Cycle:

1. Fish produce waste (ammonia)

2. Bacteria convert to nitrites, then nitrates

3. Phytoplankton consume nitrates

4. Fish eat phytoplankton

5. Cycle continues

Conclusion

Creating green water for your medaka doesn't have to be complicated or unreliable. Whether you choose the natural method for the best quality, the fertilizer method for speed, or live chlorella for instant results, the key is understanding what phytoplankton need to thrive: light, nutrients, and the right conditions.

Remember, green water is more than just algae soup – it's a living ecosystem that can transform your medaka breeding success. Start with one method, be patient, and soon you'll have a perpetual supply of nature's perfect fry food.

Once you master green water production, you'll wonder how you ever raised medaka without it. Your fry will grow faster, your adults will be healthier, and your overall breeding success will skyrocket.

Happy green water culturing!