Green-water is a type of free-floating algae that is used as a food source for raising daphnia, cope-pods, scuds, fairy shrimp, and more. These daphnia and other live cultures can then be used for feeding fish of all different sizes.
Green-water can also be used as a food source for raising some species of fish fry!
It is very easy to culture, you just need to keep it in very strong light, and every week or two remove half the green-water and add fresh water as well as a pinch of fish food for the algae to feed on. By doing this you can keep the culture going for a very long time!