I don't use UV but from other rec.ponders who
use it and really like it ---- it works by roughing
up the cell wall of the single cell algae as it
passes under the light. The mortally injured algae
then dies.
You'll still have to clean your filter pads as the
dead algae piles up, that which does not fall
to the bottom of the pond.
Bigger filters help the daily cleaning problem.

I used a 150 gallon stock tank as a filter for a
3,000 gallon pond. The media was 50 yards of
black vinyl screening which worked as a mechanical
and bio filter. Cleaned it once a year and there were
no aanaerobic spots in it.

kathy :-)
www.blogfromthebog.com