U/V Sterilisers work by killing off free floating uni-cellular algae that are responsible for green water. They also have the added benefit of killing off nasties – pathogens such as fungi, some viruses, some bacteria are killed by exposure to UV light.
The size of U/V is roughly relative to the pond volume. If you want green water clarification use about one watt (W) of U/V per 1,000l of pond volume. This value also depends on the turnover of the pond. In practical terms it is safer to aim at 2 watts (W) per 1,000l of pond volume.
For more effective pathogen control run UV at 4W per 1,000l of pond volume. You also don’t want water to flow past the U/V globe too fast so in the case of really big pond with powerful pumps (20,000l and bigger) it is often a good idea to use two UV clarifiers and split the flow half and half between them.