[Delphinus]

What I have just learned is, "everything." Even a thin film of algae on the overflow back into the tank is apparently enough to reduce its effectiveness.

I too have a Prism. I am very remiss in keeping it clean. It pulls out a very small amount of skimmate. When it was new, it pulled out considerably more. In a conversation with Rob yesterday, I complained of this phenomemon, Rob told me that he's used Prisms on small systems, and they're OK, the trick is to keep them impeccably clean. Which would explain what I've seen: since the skimmer is "clean" when it's new.

So, I will be giving mine a bath today. Perhaps even taking it outside and spraying it with the jet nozzle on the garden hose, and test this theory out.