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as far as i know they are light sources that work by using UVC rays to kill algea spores, and free floating parasites ect... in the water. they are not 100 percent nessacry, but very helpful.
A box that you pump water through ( relatively slowly ) and it flows very close to the UV bulb ( 254nm wavelength I think ). Should sterilize your water nicely, but ofcourse won't affect anything that stays in the tank, so if the parasites are on the fish then not much good really.