I purchased a 9 Watt UV filter made by Beckett. I purchased mine from
Home Depot. You'll need a diverter on the pump's output to send the output to the UV filter, which sits outside the pond, and then a hose from the UV filter will return the water to the pond.
Most people say that it takes 2 to 3 days to clear up the water. I have a 330 gallon pond and it took about 4 days for the algae to disappear. The UV filter will not do anything for algae that grows on the sides of the pond or on any fixtures in the pond. It can only kill algae that passes by the UV light. The bulb lasts about 6 months and is pretty cheap to replace. The nice thing about this UV filter is if you decide to increase the size of your pond, you can go with the 13 Watt bulb in the same filter to filter up to 1400 gallons.
-Michael