Prior to the renaming of the AC line, bio rings were not part of the setup. the sponge was the mechanical and bio component of the filter. As several have mentioned, adding a second sponge instead of the carbon is a good option. You are increasing the total bio capacity of the filter. if running 2 sponges, you can add a layer of polyester floss between them to filter out even fine particles. This will likely clog quickly, but is a snap to replace. In addition, the sponges last for years. They do not need replacing until they start to disintegrate. They are a great filter.