Ultra violet clarifiers are used for green water issues in your pond. Water passes through a chamber which houses an ultra violet light bulb. That exposes the algae that is suspended in the water giving you the green “pea soup” look and kills the algae. If you run the water slow enough through this UV clarifier you can also kill the bacteria that is in the water. Sterilizing your water is not really needed especially for ponds that have fish and wildlife in them as your pond needs beneficial bacteria to properly thrive.