How to Culture Green Water: Japan's Fry-Raising Method
To most aquarium keepers, green water is a problem, a pea-soup tank they want cleared. In Japan it can be a problem too, but it is also a tool. Breeders here grow green water on purpose to raise the fry of medaka (Japanese rice fish) and goldfish, because it surrounds tiny fish with food around the clock. They call it aomizu (青水, literally "blue water").
If green water appeared in your tank uninvited and you want it gone, that is a different job from the one covered here. This article is for keepers who want to make it on purpose.
Green water is simply tank water dense with phytoplankton, microscopic green algae such as chlorella. For fry, it means food is within reach wherever they swim. Below is how we culture it in Tokyo, and how to keep it from going wrong.
What Green Water Does for Fry
Newly hatched fry are poor hunters. They can only eat food that is tiny, and they starve quickly if it isn't constantly in front of them. Green water solves both problems at once: the phytoplankton is small enough for the smallest mouths, and because it is suspended throughout the water, the fry are never more than a bite away from their next meal. That is why, in Japan, a container of green water is the traditional nursery for medaka and goldfish. It also feeds adult fish as a supplement. And as you will see at the end, it feeds the next step up the live-food ladder.
The Two Ways to Make Green Water
Green water is just phytoplankton multiplying in water, so making it means growing that phytoplankton on purpose. There are two routes, and both work. Pick whichever suits your schedule.
Method 1: Add Concentrated Chlorella (the fast route)
Chlorella is a species of green algae, and it is sold as a live liquid concentrate you can buy online. Because it is a living product, use a fresh bottle. The method itself is simple:
- Add the concentrate to oxygen-rich water at roughly 1 ml per 10 liters (about 2.5 gallons).
- Stir it in, then set the container in direct sunlight.
Two details decide whether it takes off. First, nutrients: phytoplankton multiplies fastest with a supply of nitrate, so start with water that has already housed fish rather than fresh tap water. Second, light: sunlight drives the photosynthesis that grows the culture. Without a sunny spot, the culture won't grow.
Skip the Substrate and Plants
Grow your green water in a bare container, with no gravel and no live plants. Rooted plants compete directly with phytoplankton for nutrients and light, and the bacteria colonies living in a mature substrate draw down the oxygen your culture needs. Strip those competitors out and the algae gets the full share, which means a faster, denser culture.
Managing Summer Heat
Chlorella itself is tough; it survives even at 99–104°F (37–40°C). The trouble is what heat does to the water: high temperatures drive oxygen out and can trigger unwanted algae. As a safe ceiling, keep the culture below about 86°F (30°C). In hot weather, that usually means a bit of shade and topping up the water level as it evaporates. A reed screen or shade cloth over the container and a simple auto top-off both help hold the temperature down.
Caution: green water vs. cyanobacteria. Not every green, murky container is good green water. If the water turns slimy or thick and gives off an unpleasant, musty smell, that is cyanobacteria (blue-green algae), not phytoplankton. It can be toxic and it drives oxygen down, so do not feed fish from it. Healthy green water stays evenly tinted and does not stink; the slimy, smelly version should be discarded, not fixed.
Method 2: Let It Grow Naturally (the slow route)
You don't strictly need a chlorella bottle. Given the right conditions, phytoplankton appears on its own, and if you grow it out, the water turns green by itself. The catch is time: this route can take 2–3 months, so save it for when you're not in a hurry.
All you do is aerate the water and leave it in the sun. Used tank water with a bit of nitrate in it is ideal fuel. If you're starting from cleaner water, you can add nutrients by dropping in some bruised or damaged plant matter, or a little liquid plant fertilizer.
The payoff for the wait is self-sufficiency: once you have a healthy natural culture going, you can use it as a starter to seed the next batch, over and over. The downside is that it is harder to control (the water can foul partway through), so if reliability matters, the chlorella concentrate is the safer bet.
Keep It at the Right Concentration
A finished culture doesn't sit still. Left alone, the phytoplankton keeps multiplying until the water goes dark and over-dense, and thick green water brings real risks:
- Oxygen crashes. Phytoplankton breathes at night, consuming oxygen instead of producing it. The denser the culture, the harder that hits, and a stretch of cloudy, low-light days makes it worse.
- Fouling and odor. In warm weather, over-rich water spoils faster: ammonia and bacteria build up and the container can start to smell.
- Zero visibility. You lose the ability to see your fish, and you won't spot predators like dragonfly larvae if they move into an outdoor container.
Japanese breeders judge the right density by one rule: you should be able to just faintly make out the bottom of the container. To hold it there, do regular partial water changes to thin it, and run gentle aeration to keep oxygen up.
In a client's large freshwater tank, we once found the water so green that we could not see through to the far side, although all the fish were healthy. We changed twice our usual volume, 160 liters (about 42 gallons), cut the lighting from 12 hours to 8 and from six lights to four, and added an algae inhibitor. By our next visit, the green tint was almost gone.
For Fry, Thinner Is Safer
Fry have immature immune systems, so keep the culture even thinner for them. Thin water does mean less food, but avoiding the risks of thick water matters more, and you can make up the difference with other foods: powdered fry food, or live foods like daphnia and paramecium. Fry raised on green water plus these foods tend to grow up hardier than those on green water alone. For serious rearing and breeding, that is the standard approach: keep the green water thin and feed live or prepared food alongside it.
The Next Step: From Green Water to Daphnia
There is one more use for it. The phytoplankton that makes green water is also excellent food for daphnia (mijinko), the small crustaceans that, in turn, feed larger fry and adult fish. So once your fry have grown a little, you can put your green water to work as a daphnia culture.
Daphnia multiply fast on nutrient-rich green water, but there is a catch: they graze the phytoplankton down almost as quickly, and once the food runs out their numbers collapse. The fix is to keep a separate green-water-only container running on the side. Whenever the daphnia tank starts to clear, top it back up with green water from your reserve, and the culture stays stable.
The most useful habit is keeping a spare. Our standard approach is a bare container of green water in a sunny spot, fed with used tank water, so there is always a base to seed the next batch or refill a daphnia culture. For fry we run it deliberately thin and let powdered and live food do the rest. Food is always there, without the overnight oxygen risk of a dark, over-dense tank.