Green water is often seen as a problem in koi ponds and can take many months to clear from a newly established pond.
In an established pond the water may go green from time to time depending on the type of pond and where it is placed. A pond that gets a lot of sun will suffer from algae blooms much more that a pond that lies constantly in the shade. Some koi keepers use ultraviolet light (UV) to control the algae growth. The UV light will damage the algae cells which in turn kills them. A UV unit is usually placed at the end of the filter system prior to being returned back into the pond.
A myth often involved with UV lights is that it kills all of the harmful bacteria in the pond. This is only partly true and the light will indeed kill some of the bacteria. However the water would have to remain motionless in the UV light chamber for all bacteria to be killed. A UV light will need to be changed every 6 to 9 months if it is in continuous use. After this time the UV light will lose it's ability to kill the algae.
Some koi keepers use ozone to control algae growth. Ozone is a very unstable form of oxygen and it has strong disinfectant properties. It is created in a device called an ozonizer. In an ozonizer dry air is passed through an electrical discharge to create an extra oxygen atom which attaches to an a normal molecule. This is toxic to all life forms. An ozone unit is usually attached to the return water pipe. It is important that the ozone does not reach the fish in the pond and plants
There are also many chemical products that will help to rid a pond of unwanted algae. Remember to read the instructions before dosing a pond.