My 1000 gal pond in south Texas looked like pea soup until this week. I
bought a Pondmaster submersible UV and have had great results already. The
instructions say it should clear the water in 5 days, well, it's been 7, and
I can see clear to the bottom of the pond! I am absolutely pleased. I have
a setup similar to yours - submerged 550 gal/hr. pump to UV to submerged
filter.

Pondmaster Sublmersible Model UV20 (20 watt)
Found it at www.pond-o-mania.com for $148.95, no shipping charge

Good luck!
Craig