I can't speak for others, but I can tell you why I am down on them. First, they won't sterilize both, they can only be used for a single purpose, so pick either parasites, bacteria, or algae. The flow rates need to vary for each purpose. So you need two or three sterilizers as cell walls have different structures. Secondly, if you are fighting parasites, the most common ich reproduces in the sand. So you better hope and pray to every god imaginable that every single tomont that falls off its host makes it to the UV sterilizer before it can fall to the sand. A third reason why they are pretty useless is because aquarium UV sterilizers are greatly undersized. Reason 4, lightbulbs lose their potency very, very quickly. The idea that they last a year and remain effective, is not true. Just turning off and on the unit reduces it's life so they become an unnecessary expansive that has very questionable benefits. Reason 5, thousands of anecdotes mean absolutely nothing, as I can get thousands of reefers around the world to say that garlic fights ich, yet there is not a single piece of credible research that backs it up. Yes, aquarium UV sterilizers are fairly effective in keeping the water clean, but there are numerous, more cost-effective alternatives for that, carbon will do the trick, no to mention Ozone generators.Just my opinion and not trying to be combative.
Why are some people so down on UV sterilizers? I know the proven science for UV’s is spotty but the anecdotal evidence in people’s tank is overwhelmingly positive. Here on R2R there are hundreds of people posting positive experiences using UV for algae and Protozoa control. These same people are true believers in what UV can and has done in their tanks as they have seen the results.
Just because someone doesn’t use UV or need a UV sterilizer for their set up doesn’t mean they don’t work. If a tank doesn’t have an algae problem, need clearer water or need Protozoa control, thats awesome. It obviously doesn’t need a UV sterilizer but there are a lot of tanks that have those problems. I am quite sure there are not hundreds and hundreds of people who have invested in a UV, had the correct flow and the proper wattage and it did not work for them although there are some. Fact: UV sterilizers work if used properly in most cases and are a huge benefit to thousands of reef and FOWLR tanks all over the world. Just ask the thousands of reefers around the world who use them.
Btw, you did not seem combative at all, I hope I managed to do the same. Here are just my reasons for not having much faith in aquarium UV sterilizers. Now, if you want to toss $4-5k on a UV unit, then the conversation changes a bit.