Green water is simply a dense algal bloom, which consists of millions of microscopic life. Examples are ciliate, protozoa , rotifers , vorticella and unicellular algae feeders otherwise all known as Infusoria. Green water is really useful for feeding fry that are just too small to eat baby brine shrimp or micro worms, In the first week or two after absorbing their yok sacks. Culturing: Fill a clear bottle"container" with old tank water. Almost anything organic can be used for starting the culture, lettuce is a good medium. Boil and crush the leaves then place them in the clear container, they will absorb the water and sink. Put the culture next to a window where it will recieve the maximum amount of light possible. The whole process will take 7-14 days to turn into a bright green mixture. It's then ready to harvest. This green mixture can be used to reseed new cultures which help to speed up their development. Adding a little yeast to the mixture can also help speed things up. A cover should be used to prevent dragonflies from laying eggs in the water, as the larvae may eat small fishes or fry.