My pond is in full sun. I thought I was following all the no algae
principles but my water was usually very mucky; I would say green. I was
about to give up and buy a UV, but became suspicious when different folks
were posting about building a skippy filter and clearing their algae, which
I dont believe per se is possible. So I built a small skippy like
mechanical filter with small river rocks and a drain and it cleared in 2
days. I think, ecspecially as the season progresses some people (like me)
just have a lot more suspended debris gunk floating around in their water if
you dont have a decent mechanical filter. It your conditions are ripe for
algae growth live algae cells will slip through any filter. It would be a
shame to buy a uv and still have the same problem.

My conclusion about UV is that they are fine. If you dont want that, you
dont have to obscure all you fish for the high cover. The only down sides I
could find is 1)they typically flow much slower, which may cause you to
alter you pump/filter system and 2)Algae will tend to grow more if your fish
are producing too many nitrates. If you have a significant fish load you
may need to test your water more often for excessive nitrates, because you
do not have the 'natural' signal that you are overstocking or overfeeding.
If you decide to go UV my very unscientific observation was, (ecspecially on
the cheaper models), take the manufacturers claim for pond size and reduce
it by about 30-40 percent, particularly if you get lotsa direct sun to the
water and your pond is not deep.
HTH
Bill Brister - Austin, Texas

"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