This is a good read
I run a UV 24/7/365 on both my DT and QT
The QT is a slower flow/higher contact time setup to deal with whatever any new fish may have. I run it @ about 10gph/watt to kill everything that goes through it
This still only takes care of what actually passes through it as you can't turn over your whole tank with a little tube
In my DT I run a simple single contact 15w unit running @ around 20gph/w and takes care of algaes and such. It's plumbed directly to my chiller and keeps crap from building up in the tubes, and it helps keep my water clear
UV breaks down free-floating single-celled algaes and causes them to clump, thus they are taken up by socks so I like it
There have been times when my filter socks have built up with green stuff and I know where it came from