Hi, I have a little more info on this.

I checked the manual for my UV, and it says that the best place to
install it is after the filter.

"If an after filter installation is not feasible, your sterilizer can
be installed before the filter, but it will take a little longer to
work."

The unit I have can be used to clarify or sterilize, so that's why
it's referring to the unit as a sterilizer. It goes on to say that if
you do a pre-filter installation, you should be aware that even a
small stone can pass into the unit and cause breakage. Good point.

Also,

"Ultraviolet light does not discriminate about what it destroys. If
good bacteria passes by the light it will be destroyed."

So, the takeaways a It takes longer for a clarifier to work if it's
before your filter, there's more risk of breakage if it's before your
filter and it will kill good bacteria if it passes through. Normally
the bacteria won't be free floating like that, but if you're seeding
your pond and your filter with bacteria, you'll want to keep the UV
off until they've colonized in your filter and on the pond walls.

Maybe it's just your placement of the UV that's making things take
longer. If you really have pea-soup water, you'll need to be actively
cleaning your filter, since it should be filling up with the dead
stuff. I'm not sure what kind of filter you have capacity-wise, etc.
but that may be a consideration as well.

My first pond was installed by a landscaper who didn't specialize in
ponds. I was naive enough to believe it when he said he was an expert
on pond construction. I quickly learned more than that landscaper
after I ran into problems like green water. In the case of that pond,
the pump was undersized for the pond size, the "filter" was one of
those small pass-through filters that sits on the bottom of the pond,
and the pond was put smack under several healthy pine, sycamore, and
maple trees, which made for a memorable Fall clean-up.

The next summer, I ended up enlarging the pond (just cuz), adding a
skimmer, adding a bead filter, adding UV and tossing the pond-bottom
filter. I suppose those work for very small ponds, but I wasn't
impressed with the one I had and they're a pain to clean. That pond
was about 3,500 gallons and the filter was useless for that. Without
mentioning any names, it's the same kind of "pond filter" you can find
in your local pet store.

That pond was still a pain because of the trees, but the skimmer was a
godsend and the UV, larger pump and better filter kept the water
clear. The combination of flow, filtration and UV (if you opt for it)
is important, because the whole system needs to be matched. A wimpy
pump on a beasty filter or correctly-sized UV will not do much good.
So, try to make sure that all the pieces are spec'd for each other. I
mentioned before that I tend to oversize all these things and that's
because I've found that it doesn't take long before you have more fish
in the pond than you ever intended. Fish aren't big on abstinence.
Over-sizing the filtration and such gives you a little more time to
deal with that inevitable problem.

Anyway, I hope the extra UV info might be of some use. This is the
tough time of year algae-wise, so hang in there.

Best,

Dave