Co-Current skimmers are 100% reliant on the quality of diffuser (airstone) that is used and the strength of the air pump. This is why I recommend the Tetra Luft-Pump. It is the work horse of hobbyist-grade air delivery systems. Being pressure rated, it resists blowing out it's diaphragm, a common occurrence with other brands. This is due to the back-pressure that is exerted on the diaphragm when the airstone or diffuser becomes clogged with salt accumulation and detritus. Not only does this accumulated buildup create back pressure, it limits bubble production within the skimmer thereby reducing it's efficiency.
The Co-Current style of skimmers are great for beginners and low-capacity systems as they introduce the newbie to the wonders of foam fractionating, (I mentioned this in my last article). Also, diffusers need frequent replacing due to this constant clogging.
Tunze, and others, manufacture in-tank models that go way beyond the co-current style.
Here is an example of a Tunze Style Counter-Current protein skimmer.
