What is green water and why do you want it?
Green water is an increasingly popular resource for aquarium fish breeders, hobbyists, and aquarists. It's basically water infested with algae, diatoms, and other planktonic creatures that feed on light and nutrient-rich water. For live food cultures it becomes food for your fish food. For breeders it becomes food for fish fry and juvenile fish. Some baby fish actually eat the organisms that make the water green, others eat the infusoria that feeds on the green water.
How do you get it started?
There are many ways to start a green water culture. If you've kept aquariums for long, you've probably had your tank turn green and had a bit of a freak out. If this happens, take it as an opportunity! Grab a bit of that green water and set it aside before trying to treat the problem in your tank. This is the ideal green water starter.
You will likely get green water simply by setting some aged aquarium water in the sun. It's best if this water has moderate to high levels of nitrates to feed the algae. What's bad for your tank is often good for your algae.
If none of that works, you can simply take some yard waste and put it in water. Some dried grass clippings are a favorite starter. It's nearly inevitable that this grass has some dried out algae and other microbes that will start the culture, and the grass itself will decompose to add nutrients to the water.
How can you keep a stable supply?
Just like getting it started, there are many ways to keep it going. The three things that all these methods have in common is light, nutrients, and circulation. If you do just about anything that keeps lots of natural light (or grow lights) on the water, lots of nitrogen and trace nutrients in the water, and keeps that water moving, you'll likely have success.
The most straight forward way is to add an aquarium bubbler to your green water culture to keep it oxygenated and keep the water moving, then just keep topping it off with aged aquarium water and maybe some fertilizer as you use it up. If you are breeding shrimp or other sensitive creatures it may be best to stick with the nitrogen in aged aquarium water or, if your plants use all that up, use some shrimp-safe aquarium plant fertilizer. You can get a pretty thick soup going if you add a higher concentration of fertilizer than you would in your tank.
If you have a bigger operation going you may want to culture your green water alongside a culture of feeder goldfish. It's simple: put the goldfish in a brightly lit container such as an aquarium, small pond, or even a trash can in the sun. Keep your goldfish well fed so they create a lot of ammonia. Ensure the ammonia is being converted to plant food by filtering the water at least at the start. The goldfish will help prevent other plants and algae from growing because they graze on them, and they will also keep the water circulating because they are hyper. They keep the water nutrient-dense because they are hungry and produce lots of waste.