It is said that a UV bulb will not really be effective after about a year. Maybe even less. I have had a pump with UV bulb built in in my pond for almost exactly one year. About a week ago I noticed my crystal clear water was starting to turn pea green. Today I took the pump apart and here is what I found. It looks to me like the bulb is burnt out and you can see that the glass is heavily coated with calcium or something. I made the mistake of cleaning off the glass before taking the picture too, it was actually worse. This should help show why a UV bulb is no effective after one year.