Jerry,
The "Skippy" has nothing whatsoever to do with whether or not a uv is called for in an application. Skippys are biofilters and as such have zero effect on planktonian algea (The "pea soup algea"). Such algea appears or disappears without regard to biofilter function. In fact, an efficient biofilter produces nitrates, which are seen as food by planktonian algea. As I have noted before, EVERY pond is different, with regard to sunlight, nutrients, etc, and also the anti-algea components produced by algea itself. For many years I maintained two plant propagation ponds side by side, no filtering whatsoever, same water volume, same dimensions, just a few goldfish in each to keep down mosquitoes, and often one would be "pea soup" while the other would be "gin clear". The Skippy is a great biofilter and has been an easily constructed low maintenance answer to many watergardeners, but there is no "algea magic" going on in them. Some ponds need uv's, due to larger fish loads, more sunlight, less plant use of nitrates, etc, etc.
Love your Skippy, but don't blame it if your pond turns green, and don't credit it if it remains clear either. The Skippy does a fair job of converting ammonia into nitrates. As noted above, nitrates are algea food. Whether your pond remains clear depends on factors other than the Skippy.
For a very incisive discussion on the causes of green water, there is a paper written by Norm Meck, published on the internet by the Koi Club of San Diego. Everyone who is interested in planktonian algea should read it.
See it at
http://koiclubsandiego.org/library/green_water.php
