I have had exactly this situation going on with my pool since July of 2004. In my case I blame it on my Lowzone systems having an excessive airflow rate resulting in continuous aeration. I recently choked it way back so the I get much less "champagne bubbles" in the pool. I figure the UV bulb can't be producing that much ozone, so excess airflow is just diluting the what ozone is being produced and making it less effective. I know this site is not real keen on lowzone systems to begin with, so I'll leave it at that.
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