I would think that there is some minimum level of DOC necessary for foam production, below which the matter is re-desolved at the surface when the bubbles burst.
As far as two of the same skimmer type operating differently, I can only guess that it would be due to different inlet compositions relating back to the installation. As was mentioned before, if one was receiving some of the water that had already been through the first skimmer then the foam production would be less.
Depending on the amount of skimmate you make, you might want to consider bagging it and selling/using it as fertalizer. On a 5000 gal tank it mught be a viable option.
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