Green water in garden ponds and small lakes

Water turning green in garden ponds and small lakes tends to happen in the summer months when the water heats through the action of the sun plus most garden ponds and small lakes are quite shallow which means they heat up quicker than large bodies of water such as larger lakes.
In addition, high levels of nitrates in the water accelerate the green water effect which is an algae bloom. In warm water with lots of available nitrates, algae grow quickly.
The best cure is to add barley straw stuffed in an old pair of tights with a couple of rocks or stones to hold them down in the water. Add the tights to the pond etc and secure them to the bank with some string. 
The bacteria and other organisms try to break down the algae causing the green water as a food source and in doing so use up all the nitrates in the water causing the algae to reduce.
In addition, adding more pond plants to the water will help such as lilies, Elodea Densa (ANACHARIS ) will help to keep the water cooler stoping green algae from forming 
In a new pond or small lake green water is quite normal whilst the water reaches a balance especially if the pond or small lake was originally filled with tap water.