A UV clarifier will not get rid of string algae. Remove as much string algae as you can from the pond then use an algaecide such as algaefix. Be careful, read the instructions and use the correct amount. Add it in the morning. I have been using it for years and it works great. It will take about 4 applications. After the second application, the string algae will start turning white. By the fourth application it will dissolve.