Prevention is better than cure. Sort out the reasons why you get green water (too much light or nutrients) & you don't need UV.Green water is an imbalance where higher forms of plant life are not present to take out nutrients.
UV is good for killing free swimming stages of some parasites but is not the best thing since sliced bread. I put one of the first commercial centralised filtration plant for freshwater fish into a UK importers & know what they do or don't do.
You need to replace bulbs at least every 6 months or the bugs just get a nice sun tan passing through.
Have you tried dried banana leaves to increase tanin level?
Personally I would forget UV..........