I can't speak for Guy, but the only time I would recommend a UV would be for possibly contolling green water issues in ponds. I have 2, I use neither, the last time I used one was to clear up a bacterial bloom I was having after fricking around with tank parameters and causing a spike, about 7.5 years ago. It was unnecessary to use it then, I simply didn't want to wait a few days for the tank to stabalize and take care of it themselve. I do believe they should be used! In the LFS and the wholesalers, but in units that are huge!!! Guy mentioned a 110W UV, this IMO is still to small to be effective at flow rates for a average sized aquarium. I'm talking aboout the 6-8 foot 200-300Watters, those are effective at reducing parasites and bacteria and viruses, but only it they go through them. In a home aquarium the stress level of the fish should be able to be maintained at a low enough level that they remain healthy.