| I sell these UV units at my shop. 90% or so are sold for the sole purpose of pond water clarification. Some of the newer models have elaborately designed helix's within the unit in order to increase the exposure time. UV can be dangerous to people, but you have to be very careless to get a dose that would do more then give you a very bad sun burn. The UV lights can damage the retina of the eye. UV light damages the DNA of cells, this is how UV unites clarify or sterilize water. High doses of UVA radiation can destroy some of the function of algae and therefor is used in clarifier's. UVA,B,and C radiation can and does change the structure of DNA with in cells and is used in "sterilizers" the cost of the glass that allows B and C radiation through accounts for the higher cost of the bulbs. | 
