Carbon is handy for the removal of certain thing such as medicants, but is not necessary for regular use. As well, it depletes rather quickly. With Ac
filters, the sponge is a bio media as well as the ceramic tubes. The sponges will last for many years and only need to be cleaned regularly by squeezing the debris from them in a container of tank water, when doing water changes. The ceramic tubes should last pretty much forever. That is the beauty of AC
filters, as once you have them you have no more ongoing media expenses.