More or less. UV is a very short wave, non visible light that causes you to
suntan, and will cause cancer due to overexposure. It will also burn the
retina of the eye. It is this principle of burning living tissue that
allows it to kill algae, bacteria, and other living organisms in the water
that pass through the light. The main reason that we use the UV is to kill
the green water algae. The string algae and the sweater algae do not pass
through the light and are unaffected. The good bacteria in the pond are
attached to filter media, pond liner, piping, plant roots, etc. and as such
are not exposed to the UV. The rate of flow has to be slow enough to give
the algae a big enough dose to kill it for the UV to do its job.

UV bulbs are kind of like fluorescent bulbs in that they lose intensity with
time. The intensity of a UV bulb can be reduced by as much as 60% in one
years continuous usage. If the flow rate through the unit is near max, then
as it loses power, the ability for kill is reduced, and therefore the
manufacturers recommend replacement once per year. If you only use it for a
few weeks in the spring, then it should last several years. If you have a
flow rate 50% as high as specified through it, it should still function even
if its intensity is lowered to 50% and maybe even to 40% which would take 2
full years or more.

--
RichToyBox
http://www.geocities.com/richtoybox/index.html
"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