I found this, on WWMedia

"Copepods and UV Sterilizers
Hi,
Will running a UV sterilizer kill beneficial copepods and amphipods?
<If it is powerful enough, the flow rate through it long (time-wise) enough, yes>
I had a large bug explosion a few months ago so I got a dragonet and he was eating like crazy. Now, he still looks like he's constantly picking through the rock and sand, but he is getting skinny as if he can't find enough to eat. I'm thinking about setting up a refugium under the tank and getting a amphipod breeding mat and starter culture from Indo-Pacific, but want to find out why the visible bugs are gone? (Still see larger bugs from time to time.)
<Likely consumed by the Mandarin/Dragonet... they can/really mow through such fauna. I do agree, urge you to go ahead with your added sump/refugium plans. Many benefits, much fun. Bob Fenner>
Thanks for your time,
Michael"