Co-Current, Counter-Current, Needle Wheel, Venturi; it all sounds pretty complicated, but contrary to what some people would like you to believe, protein skimming ain't rocket science. Skimming is no more than injecting air bubbles into a water column, twirling them around for a bit, then skimming off the crud (proteins) that they collect. Exactly how different protein skimmers do this is a bit of an art, however. Here are the simple nuts and bolts of protein skimming. Understanding the principles makes it easier to find the right skimmer for your saltwater aquarium.
More: Top In/On Sump Protein Skimmers Compare prices on these state of the art sump mounted protein skimmers.