skimmers use air, broken into as small of bubbles as possible to remove proteing, the foam forced over the top of either a tube or cone shaped body, into a collection cup is the purpose. The air is injected in multiple ways. Pin/needle wheel pumps both water and air into the reaction chamber. Another popular type uses Spray Injection. High preasure water is Sprayed into the reaction chamber, taking air bubbles with it. Think of a Hose spraying into a soapy water bucket. Some will swear by needle wheel, some by Spray Injection. Truth is, they both work. THe method of air injection is not nearly as critical as the Brand of the skimmer. This isnt the area to go cheap. A high quality skimmer will be very useful and work well...a cheap skimmer will do little, and give you headaches. Popular Brands are
Aqua C
Reef Octopus
Bubble Magus
Bubble King
and other.
Stick with one of those brands, and you wont get taken. And remember. The manafactured recommended tank sized, if possible, go 1.5-2x the volume of your tank. if ya have a 65g, get a skimmer than can handle 120.