A UV is a bulb inside a PVC tube that is protected from water by a tube of glass. Water flows through the PVC tube and passes by the long bulb so that the ultra violet rays emitted from the bulb can sterilize microscopic algae that flows along with the water. This prohibits reproduction and growth of the algae and it quickly dies back. A UV is not effective against string algae because this type of algae attaches to the sides of the pond and rocks in the waterfall and does not pass through the light.