It will only kill the free-swimming larva, not the parsite that is on the fish, which is only a portion of the entire life-cycle and then it needs to be very strong. A good portion of the larva won't even be pulled into the UV sterilizer.
Best treatment for Ich is QT with hyposalinity and a fishless cycle (2-4 weeks) of the DT. The Ich will not survive without any hosts in the DT,
Hope this helps.