Like pretty much everything in this hobby, there is no absolute answer. There is no doubt they do work, I don't think they erradicate everything (good or bad), the only way to catch everything would be to run the return pump through it and most people's flow rate is too high to kill anything, let alone everything. There are charts with most UV units to tell you the recommended flow rates. I have a 36 watt on my 180 LPS/Softie and no UV on my 90 SPS tank and I can honestly say I have no issues with either tank. The fact of the matter is do you NEED a $5K light fixture, a $2K BK skimmer, or a UV? They would all be nice to have but you can do just fine without them, and good husbandry is better than any of these things.
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