I have had these for years they last a long time, like 20 years and really do keep the pond clear, you do need to change the bulbs every 6 to 9 months.
Kockney Koi Yamitsu UV Units (Clears Green Water) 11w
I've been using these clarifiers for 15 years & never had an issue with them. Easy replacement of UV tube & the quartz sleeve is easily removed for cleaning. A very good value & quality product.