Koisrus,
I'd wondered the same thing, and found lots of opinions on this. I decided to install a 13 watt uv after the skimmer and pump and before the bio filter. I did this thinking that the clumped algae killed by the uv would then be caught by the bio filter before sending it back into the pond. I installed a by-pass tee controlled by a valve in my main 2" line, and divert about 60% of the flow through the uv, then back into the main line into the filter. In less than two days my water went from cloudy / green to crystal clear. You can literally see every single pebble at the bottom of the pond, etc. My pond is fairly new so I expect that once the bio filter becomes established I will likely close the valve and shut off the uv most of the time. No doubt there are lots of different ways of installing one, just thought I would share what has worked for me. Good luck!
James