The reason I am dubious of any observable effect is I moved my current UV from my planted 65 gallon to my 120 gallon discus tank. According to the charts and my flow rate, I should be killing all algae, bacteria, and parasites and I change the bulb every 6 months. I have not noticed a difference in either tank after a year. When I got discus from Kenny and after QT combined them with my stendkers bacterial hell broke out in my tank (documented in a thread here) and I almost lost all my original fish. UV did not prevent or seem to help that situation (a series of PP baths, Kenny, and my friends and simplydiscus saved the day).

Maybe it is less helpful for me because I already have low pH, low TDS water out of the tap in addition to frequent water changes. Logically, pathogen killing light seems to be common sense. Anyway, I would not spend the money again, but I am glad to hear these things are more helpful to others than it has been for me.