Try adding calcium to fresh water and then try a protein skimmer.
Call me nutz but I have seen whirlpools running with no bubbles then by adding fine calcium the whirlpool begins to lather up. I guess It would be an neat experiment in a empty tank.
Kevin

I have seen in waste water treatment plants use rubber type membranes that diffuse air into the tanks. They generate a fine enough bubble for fractionation purposes.

Schuran makes a FW skimmer.

But really, and this is strictly IMO, why would you need a FW skimmer? Are FW tanks not easy to maintain..?

Large aquaculture systems and ponds benefit greatly from protein skimming, particularly when water changes of a large size are so time consuming. Of course the intent for protein skimming is to improve water quality overall on a constant basis, which any aquatic environment, even a freshwater 10 gallon aquarium would benefit from.

Big Al's BC

Which is beyond what I'm trying to say. Would you, personally, spend a couple hundred to a couple thousand extra dollars to keep your 10 gallon tank a little cleaner?

Ummmm....not AT ALL true. Plants NEVER EVER output CO2. They USE CO2 to create O2, however with no light, there is no photosynthesis, therefore no O2 output from plants.

http://van.physics.uiuc.edu/qa/listing.php?id=848

I'd be tempted to say that plants are constantly outputting co2. During the day time it's outweighed by the output of o2 from photosynthesis.

Sorry for the off topic.....