Quercus Robur wrote:

I am sure this has been posted many times, so please excuse me.

A while back I saw a post about using UV filters to remove algae. This
puzzles me. Algae thrives on UV from sunlight,

Algae thrives on light, period. I've never seen anything to indicate that
it's particularly UV, but...

so how do these filters differ?

They're a whole lot more powerful. Sunlight generates something on the
order of 100 Watts/meter squared - spread over all wavelengths. A UV lamp
will generate 10s of Watts over a small fraction of a square meter - but
almost entirely in the UV spectrum.

How efficient are they? I have a 120 gal pond and am
considering a filter which combines a pressure filter and UV clarifier.
Nurserypro Ultraklean 500 gal.

they're very efficient, but imo not a good idea. When you have an algae
problem, it means you have too much nitrate, phosphate or light. Sometimes
it means you have nitrate that can't be used by vascular plants because
they don't have enough of something else (frequently potassium). Killing
the algae with a UV does nothing to resolve that problem - it just makes
the water clear.

My present filter is a pair of foam rubber filters in plastic boxes 6x3x3
in and I have to clean daily.

You won't have any less need to clean a filter that has UV, so if it's not
bigger than your present filter, you'll still need to clean it often.
--
derek