| UV bulbs have a predictable deterioration of about 8000 hours for almost all bulbs. This happens to be the close to the number of hours in a year but down time will extend the 8000 hours longer. Some fixture manufacturers claim more, but I haven’t bothered to check to see if it’s because of improvements in bulb design or just ‘improved’ specks. The manufacturer 8000-hour bulb life statement is developed from the accepted standard level of 80% of a new bulb output unless their posted specks say otherwise.
On the green water. With adequate filtration and proper stocking, the biofilter and reasonable shading should be able to handle spring startups after the two or three years it takes the filter to mature. That should happen sooner for all but the late spring, early summer months so the bulb should be able to be turned off most of the year, extending bulb life accordingly.
254nm germicidal light deteriorates rapidly in the first few inches of water so a fixture over a sump fixture would have to be much larger, to be as effective, costing more to replace bulbs and operate at a time we should be looking for ways to conserve energy. The fixtures most of us use allow more water to come within a 1" range of the bulb which allows for a very intense exposure as long as the sleeves are clean and the suggested life is respected.
I install UV fixtures at the end of the filter string so the quartz tube won’t foul as fast. Still, the tube fouls quickly as the slime coating develops and should be wiped quarterly if you want maximum output.
Practically, we seldom install a fixture that isn’t overrated for our needs to eliminate the pea soup. That reduces the effect of tube coating and aging so the bulb life can be extended in our ponds. The UV levels we use aren’t powerful enough to kill most bacteria and aren’t nearly enough to impact parasites. They’re only installed to keep our water clear.
It all boils down to installing a fixture rated for the pond, cleaning the quartz sleeve regularly and replacing the bulb when it gets old.
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