A word on flow with these things as it will answer many of the misconceptions that are being posted in this thread...
UV units have to have very slow flow through them. No more than about 5gph/watt. If you pass water through them too quickly, the things you really want to kill, such as ich, will not have sufficient dwell time (time exposed to the lamp) to be affected.
UV units will kill beneficial bacteria going through it, but these bacteria breed far more readily than the bad stuff that is being wiped out, so your system will adjust. Also, remember that most of the beneficial bacteria population lives in the sandbed and on the rocks, not free floating in the water column. Finally, most of the time the flow through the unit is only a fraction of the total system flow, so it's not like it kills all bacteria going through the filter.
Plumbing a UV unit directly onto the return pump or in any other scenario where there are hundreds of GPH passing through it will only serve to drive up your electric bill and give you a false sense of security. You want to set it up with a dedicated loop off your sump driven by a powerhead or something, or with a "T" off your return line and a ball valve for regulating the flow through the unit.