The Skimmer is a reaction chamber for the Ozone. The Charcoal is to make sure all the ozone is filtered out of the water and escaping air. Ozone helps brake down organic wast - kills off parasites - and agile. That set up has cleared the tank up nicely. The skimmer skims nothing but it a nice reaction chamber!
There are a few long post I can link if you want more info on Ozone in freshwater.