I am with Jerry, I currently do not have a UV light on either of my ponds. 5 foot deep pond at home has a diy sand and gravel filter and you can see right to the bottom, and my old preform that is now installed at my work has a small skippy, you can see right to the bottom as well. But just as Jerry stated, there are smarter people than me out there that swear by their UV lights...
Greg