I had mine on since day one about 2 years ago. The tank has ICH about 9 months to a year ago, even after 90 days of fallow prior, cos I QT fishes but not coral. IMHO Corals can get their nutrients from fish wastes and feed, and my tank has plenty of them. Both my ATS and Refugium are overgrowing requiring a trim every 2-3 weeks. Everything is a swell!
Till the very moment I decide to turn the UV off totally and Skimmer to 8 hour daily, in favor of biopellets and bacteria seeding/dosing… ICH emerge with a vengeance and within one and a half month, ICH took out a two third of my fishes. I have had my UV up 24/7 now for past 2 weeks, I’m still battling ICH as we speak not sure how many more fishes are to succumb to ICH eventually. I may have to restart my 132G.
FWIW, keeping bacteria in the water column as coral food being natural over running UV 24/7 is a fallacy; nothing we do in reefing is natural.
While we are on the topic of UV…I picked the turnover of the total water volume that I wanted, in my case it would be 3X of 132G, which was 396gph. Then I picked the UV with the manufacturer's rated parasite killing power, DELTEC 80W. And I ensure that the throughput of my COR20 return pump NEVER exceed 396gph with the APEX flow meter, in fact I ran with a 10% safety margin.
Hope the above helps.