Uv (ultraviolet) clarifiers can clear green water pea soup colour in a very short time. UV clarifiers use ultra violet radiation from the lamp to kill the suspended algae making the algae cells clump together so that these dead algae can be mechanically filtered.
When UV clarification is used in combination with a filter UV lamps inside the unit have to replaced every 12 months.
You can try to control algae with pond chemicals , but for a permanent solution to the problem use a UV clarifier.
lamp sizes range from 5watts, 7watts, 9watts, 11watts, 18 watt, 24watts, 36watts, 55watts.
you can pull the lamp out of the unit or filter, the size will be shown on the solid end of the lamp, if its hard to read, look at the brand of filter or UV unit and lookup the brand and model that will tell you the size of the uV lamp you need.