In simple terms, the UV light doesn't do anything to the water. It will,
however, kill algae cells as they pass by with the water. Hence they will
reduce the algae in your pond IF the UV light is sized correctly for your
situation. The bulbs need to be replaced about once a year because even
though they still light up, the amount of UV light they produce and thus
their effectiveness, falls off over time.

Paul

"Mostyn" wrote in message
...
Can anyone tell me what does a UV light actually do to the water & why do
you have to change the UV bulb/tube and how often do you have to change
the
bulb/tube.
Thanks
Mostyn