I knew someone would ask that.
Using a jar with a lid, fill it with water and put carbon in the water.
Using airline tubing, feed the line from the air pump (or free air) down through one hole in the lid of the jar to the bottom of the water.
Then feed the airline that goes to your skimmer into another hole in the lid, above the water.
This makes all of the air that enters the skimmer go through the water in the jar. The carbon filters the water and cleans inpurities out of it. Same concept as a bong, only for a much better use.
(debatable)