I've noticed a dramatic difference with bulbs that are old vs. new,
and the material I've read does say that the bulbs become ineffective
after about 14 months of use (so maybe every two years). Yes, they
still light up, but they're not doing what they're supposed to be
doing. My experience backs that up. I expect that the bulbs don't just
suddenly become ineffective one day, but rather it's a gradual decline
over time. Replacing them earlier may not strictly be necessary but it
can improve performance.

My UV sterilizer has wipers on it, and the chambers are most
definitely not filling up with algae. Rather, the bulbs themselves
become less effective over time. However, in a situation where the UV
is before the filter, I think you could have a major "gunk up" problem
that would make the unit ineffective.

The "sterilizer" vs "clarifier" difference is only based on the size
of the unit compared to the size of your pond. I agree you don't need
a unit acting as a sterilizer, but a sterilizer is just the same
"clarifier" unit, handling a smaller water capacity. At least that's
how the Aqua Ultraviolet units are.

You're right, the UV doesn't kill the algae, it sterilizes the free-
floating stuff. It kills smaller things that pass through it such as
bacteria, viruses, fungi and so on.

I'm surprised to hear that you need UV with a vegie filter. Isn't the
veggie filter alone enough to control the algae or do you use the UV
for other reasons?

- Dave