Lotta different thoughts on UV and it's effectiveness (or ineffectivness or over effectiveness)...Depends on what you want to kill (determined by flow rate and wattage)...
But that misses half the water.
Doesn't matter where you put it...eventually it will process all the water...again, flow rate and wattage affect kill rate...
Personally, I don't have one on my reef...do have one on my FOWLR and it's kind of hard to tell if it's effective or not, because the two systems are so different...I'd have to take it off the FOWLR to see if anything changes....Things are going well (no parasites and minimal *no* algae) so I'll let it run...