Green water is caused by millions of single celled algae, these are so small they pass straight through conventional filter systems. If your pond already has a pump and filter the easiest way to get clear water is to add an ultraviolet clarifier. As the water passes over the UV light the algae is killed, once these are added to a filtration system the water normally clears within a couple of weeks and then stays clear all year round. These units start around £50.00 and cost around £10.00-£20.00 a year to run depending on the size of your pond. If your pond does not have a filter a good growth of plants especially lilies & floating plants is a good way to keep the water clear, these provide shade & compete with the algae for food. As you may also be aware there a many treatments on the market for dealing with green water. In our experience the most effective ones are: Waterlife Algizin G if your pond is filtered. This works by clumping the algae together so it is large enough for the filter to remove. For unfiltered ponds try barley straw, although this slow to work. This is due to the fact bacteria need to grow on it that control the green water so allow a month for it to work.