Sometimes a UV is a great thing. (I was lucky and found mine on
clearance for $39 down from $119--Whoo-hoo!)
Here in Arizona, my pond is crystal clear, with tons of plants, good
bio filtration, and a moderatly stocked fish load. Looks great, lots
of lily pads to cover the surface. This is fine, until...Arizona
Summer! Since we live in a cookie cutter community with a Gestapo--I
mean, Homeowner's Association--and a very small backyard, we don't
have any shade trees or structures we can put up for shade. Once we
hit summer (~110 degrees average), lily pads don't last very long and
I go from about 75% coverage to about 30%, with lots of strong sun and
nice warm algae growing water. Until I started using the UV, I had
murky water until about the end of September, when it cleared up
again. I don't think it's so much the heat as the strong sun that
kills, oh, anything that is not desert native! It just burns up the
leaves. My marginals do pretty good, but the lilies don't really like
it. They thrive in spring though. And I'm sorry, yes it's not 100%
natural, but I want to see my fish! Plus the warm water encourages
parasites, so it helps keep them out. All in all, I'm very happy with
it.