Thats what I thought. What youre refering to is more of a "supplimental" filtration. Bio balls and such have proven themselvs to be not as effective for filtration as live rock, >deep sand bed< (major discussion on this one) a good skimmer and regular water changes. The wet dry filtration systems tend to build up nitrates over time and become detrimental to the filtration process. Most people nowa days are using the live rock, skimmer method with tremendous success. This method is considered "biological filtration"