In order to maintain a clear pond or aquarium, you need to match your UV to the volume of water with the correct size pump to gain maximum efficiency at preventing algae growth and green water.
POND UV Sterilizers
8 Watt U.V - pond cap. 6000L - Max flow 2400lph
15 Watt U.V - pond cap. 12000L - Max flow 3600lph
30 Watt U.V - pond cap. 30000L - Max flow 8400lph
55 Watt U.V - pond cap. 50000L - Max flow 18000lph
Profesional UV Sterilizers
( the below-mentioned volumes are for medium stocked marine aquariums)
8 Watt U.V - Volume 100L - Max flow 600lph
15 Watt U.V - Volume 200L - Max flow 1200lph
30 Watt U.V - Volume 500L - Max flow 1800lph
55 Watt U.V - Volume 1000L - Max flow 2400lph
https://www.ultrazap.co.za/uv-clarification.html#tube-maintenance