19/10/2026
Why "Green Water" is liquid gold for Medaka fry 🟢🐟
If you’re new to breeding Japanese Ricefish, your first reaction to seeing a tub turn green might be to scrub it clean. But when it comes to raising tiny, freshly hatched fry (hariko), green water is your ultimate secret weapon.
When Medaka first hatch, they are incredibly tiny—just 4 to 5mm long! In Japanese, they are called hariko (針子) or "needle children." At this stage, they are far too small to consume standard micro-pellets or flake food. Attempting to feed dry food too early often leads to uneaten particles sinking, rotting, and fouling the water chemistry—the #1 cause of sudden fry loss.
Here is why green water works so well:
1️⃣ Constant Micro-Feeding: Green water is packed with single-celled phytoplankton and microscopic live organisms. Because it’s suspended everywhere in the tub, the tiny fry can graze non-stop throughout the day without having to compete or wait for feeding times. This dramatically improves their survival rate during their most vulnerable first two weeks!
2️⃣ Natural Buffer & Reduced Stress: The natural cloudiness of green water provides built-in cover from overhead shadows (like birds or sudden movements), keeping young fry calm, active, and less stressed near the surface.
3️⃣ Zero Water Pollution: Natural sunlight keeps the algae active. Unlike dry feed, living micro-algae absorbs nitrates and ammonia while producing oxygen, keeping the water conditions stable.
How to get the right balance:
You want a light, tea-like green tint where you can still see an inch or two into the water—not a thick pea-soup sludge. Plenty of natural morning sunlight and a small pinch of organic nutrients is all it takes to keep a culture going outdoors.
Once your fry reaches 2 to 3 weeks of age (approaching the juvenile stage), they'll develop better swimming ability and you can gradually transition them over to powdered fry foods or live cultures like baby brine shrimp.
💬 Do you intentionally use green water for your fry tubs, or do you prefer keeping them in clear water? Let's talk about your fry setups below! 👇
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