almost got it all :)

[o.c.d.]

They fry beneficial bacterial and plankton populations in your tank Furthermore they can raise the Temp of you tank. Don't get me wrong they do sterilize the water and reduce pathogens but all the water in the tank would have to pass through it and that just doesn't happen. Leaving refuge areas for reproduction of pathogens ect. If proper water quality is maintained and you try to buy fish that are healthy. IMO a UV unit is not needed.