I was just re-reading the original post of this thread, and it makes
me wonder if you're pump/filter combination is really up to the job.
If the filter isn't doing enough, UV isn't going to help.

What kind of filter is it, and is it in-pond or out?

Before you changed the water, did you do any water tests? Those would
undoubtedly show excessive ammonia or nitrates, like Ingrid said. I'd
guess with so much algae your PH is climbing too. That's something to
be careful about if you're doing massive water changes since you could
cause a wild flux in PH.

It really sounds like your filtration isn't enough to handle your fish
load. Are there a lot of fish in this pond? If so, you'll need beefier
filtration unless you can remove some of the fish.

Dave