Green water is a generic term for water with suspended algae or euglena . Euglena are small single cell critters that share traits of both plants and animals. They are eaten by larger animals up the food chain.
Some very small fish fry or fry with small mouths require very small food. I add a small amount of green water to tanks with fry with very small mouths like some of the smaller Epiplatys, Procatopus, and Nothbranchius species for the first few days. It won’t hurt, and seems to help the survival rate. Mostly though, I use green water to feed other critters like daphnia and brine shrimp, which are then fed to the fish.
I start with a gallon pickle jar and put in about ½ fresh water and ½ clear aquarium water. If your water has chlorine or other chemicals, let it sit for several days before using it. Don’t use aquarium water if you have any sick fish in that tank. I add a pinch of Miracle Grow fertilizer. The aquarium water is used to provide micro nutrients. Add a cup of green water from another culture. If you don’t have a starter, place it in the sun, and it will probably start to turn green on its own.
Place the jar in a sunny window. If you don’t have one, you can use artificial light. One of the 60 watt compact fluorescent bulbs produces a lot of light with minimal affect on your electric bill. Put it on a timer for about 15 hours per day. Depending on what you have growing, you might see a green settlement on the bottom. If you have Euglena, they will tend to settle out. It is not a bad idea to stir the jar every day or so. I like to put in a couple of ram’s horn snails to eat algae forming on the sides, which blocks light to the suspended algae that you want.
The water will get dark green in about 10-14 days. Replace the water you use with fresh water. If you let the culture go too long it will turn yellow and crash. To prevent that, it is best to dilute the culture when it gets dark green. Save a few cups, and dump the rest. Use the saved green water to start the new culture.
You can also raise green water outside during the warmer months. Start with a white 5 gallon food grade bucket. Fill it up with fresh water. Let it sit a few days if you have chlorine in your water. Add a teaspoon of soluble plant fertilizer and add a cup of green water. Stir every day or two to keep the algae suspended. Once a week I pour off about 2/3 of the bucket into my daphnia pools. Fill the bucket up with fresh water and a little more fertilizer.