Tunze "contact" skimming vs countercurrent mode

[Who me?]

So I'm interested in the 9004 DC for a small 30 gallon I'm setting up...
I noticed the new comline series skimmers are now set from the factory in contact skim mode with the impeller cover off. Does anyone know the advantages/ disadvantages to running it in contact mode as opposed to countercurrent mode?

From what I can gather, countercurrent mode would provide surface skimming but would also be more likely to introduce microbubbles from the bottom of the unit. But I'm sure there has to be more differences??

Thanks!

 

[rvitko]

That is really the main difference. In contact skimming, waste diffuses in by osmosis, this method of skimming is plankton safe for this reason, but it is slower to respond to waste, it will generally remove the same amount of waste but not as rapidly.