Waste of money imo....unless you have a green water problem. Won't do a thing against parasites.
That isn't exactly true though it's impact isn't great when it comes to parasite control. It can reduce the spread of parasites in their waterborne or free floating stage if properly set up with appropriate flow rates but will never eliminate them. The problem with parasites such as ich is that they host on the fish and are only in the water column for a very short amount of time. They wind up in the substrate before finding a new host. As such, a UV will never eliminate the parasite.
As for being useless, I will disagree with you there. A quality UV that is properly setup will improve water quality. Not only clairity but also the ORP. UV rays will oxidize organics rendering the water clearer and cleaner. Some people use ozone for similar benefits. It will also reduce the spread of cyano bacteria, prevent bacterial blooms and virtualy eliminate algae from the water column.
I use AquaUV and have for many many years. I run a 114 watt unit on my main display. A few weeks ago, I noticed my ORP had dropped nearly 100 points virtually overnight. My display is 480 gallons and has over 60 heavily fed fish. It's a very high nutrient reef. After a little investigating, I realized my UV sterilizer bulbs were dead. I normally replace them ever year but screwed up and had gone over 1.5 years. I replaced the bulbs and within a week, my ORP was back to it's usual 380+ ORP. I have both Apex and Profilux on my system and tracked the results across both probes. My Apex probe was replaced about 2 months ago. With an understanding of ORP, regardless of how important one may think it is, it is about the only measurement we have of water quality and the difference between 275 and 380 is significant in terms of water quality when it comes to ORP. That said, if you're going to get UV, get a quality one that is properly sized and setup with proper flow. Otherwise, it absolutely is a waste of money.
As for results, these graphs speak for themselves. This graph is what prompted me to investigate and this is what led me to realizing my bulbs were out. And yes, it's a big sudden drop but given the fish population in my tank as well as my feeding habits, UV sterilization plays an important part in my systems water quality. Even more important that I realized.
It took me about a week to get the new bulbs. This is the result from earlier this month. The ORP began climbing immediately after the bulb swap. Within a week of changing my two bulbs, my ORP went from a low of 275 back to it's normal range and is now peaking at around 390. No ozone on my system. upwards of 70 fish, fed 3x a day with an overfeeding at night.
And today.