I have a 40 watt UV clarifier hooked up to one of the gravity-fed
outflows from my filter. The pond is approximately 4000 gallons. For
the first month of owning the thing the device gradually made the pond
clearer and clearer, then last weekend it got so clear that I could see
the bottom of the pond which is at about 4' in depth. I was very happy
with this, and even commented to several people how it was great to own
a product that claims to solve a big problem and it actually works. I
work with computer software so I've grown very skeptical of promises
since in the computer world things fail far more often than they work,
so when this clarifier resulted in water that was this clear I was just
amazed.

Then Murphy's Law kicked in.

In a matter of maybe 3 days the water clouded back up to a point that's
nearly as bad as the days before the clarifier when in. Has anyone
ever heard of something like this happening? I can't imagine alge ever
being able to build a defense to intense UV-C radiation, so I don't
think this is an example of the alge building a resistance to the
light. So what the hell went wrong?

Around the time that the alge returned I cleaned the filter, cleaned
the pump, and put the pump into a new protective housing. Here are
some thing that have happened in that time that somehow may play a part
in this stupidity:

1) During the process of cleaning off the pump and putting it into a
new housing, I forgot to turn off the UV light. Could being on for
about a half hour to 45 minutes without any waterflow cause it to
become damaged? I can see from the clarifier's glowing housing rings
that the light is still on.

2) After cleaning the pump, its output increased dramatically. I had
one of the filter's 4 gravity outflows intentionally disabled, and the
filter began overflowing so I restored that 4th outflow. I thought
that may have resulted in less water flowing through the UV but since
the total amount of water pumped is higher plenty of water should still
be going through the UV. Since the total amount of alge was near nill
anyway, the alge can't reproduce much so reduced waterflow shouldn't
hurt anyway.

3) The temp last week was a few degrees warmer than in previous weeks,
and the sun is getting more direct.