Yeah, when you have it running make sure the water CRAWLS throough it.The slower the flow rate, especially under the rated minimum, the more dwell time the water will have under the UV rays/lamp.In turn this will effectively blast more nasties like bacteria/parasites/algae.
I have the turbo twist 18w on my 220. They list the flow rate on the box. The only rate I remember is the lowest to kill parasites - 120gph. I've got mine down to 140gph. I run the fluidized bed from the same pump so it's a dance to get in high enough for the FB and low as possible for the UV.