"Lee Brouillet" wrote in message
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UV can *only* kill that which passes through it. The biobugs
growing in the biofilm growing in your filters is static: it doesn't move,
is not exposed to the UV, and is not subject to annihalation by it.

Considering a NEW filter with no biofilm. Water flows past it. If the water has
lots of nitrifying bacteria then the new biofilm will grow faster. The bacteria
come from the other surfaces in the established pond.

Whether the effect is significant is the question. Does a UV make establishing
the filter take an hour/day/week longer? Certainly the effect is less if the
water coming from the filter is UV'ed rather then the water going to it. But again,
I wonder how much.

Since it costs nothing to leave the UV off while establishing a new filter, that
is what I would do.