How much surface agitation do you have? I had a section of my sump where I had nearly no surface movement what so ever. The water flowed so smoothly over the baffles that there was no surface movement and I found I had a slight film develop over it in time. I just assumed it was an accumulation of something, either protein the skimmer didn't get a chance to remove or a byproduct of one of the tanks inhabitants.
All I did was move a piece of equipment that roughed up the flow before a baffle creating some wave action across that section and I've never had it again.
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