One of the problems with cycling UV on/off is thermal expansion and contraction of the air surrounding the lamp. I have multiple tanks, all running UV, some run 12/24 some 24/24. The units that cycle off for 12-hours tend to draw in much more moisture/water around the quartz sleeve seal and into the lamp chamber.FWIW -
AquaUV kinda dodged the question but said "we designed the unit to run 24/7 and that is our recommendation".
I don't really see any issues running it 12/hrs per day and your probably get 2x the life from the bulb....
That may not be an issue for AquaUV or other brands of choice, but so far it has been a real factor in longevity of my units.