Yes, mud. The latest eco-system for filtering marine tanks is to use a separate tank under or connected to the main tank. In here there is a mechanical filter which removes large particulate waste. The clean water is then passed over a live mud bed. Growing in this is Caulpera seaweed. The combination removes and processes the waste materials from the water. This is then passed through another mechanical filter (to stop fronds of caulpera) before being pumped back to the main tank.
Heaters can also be included in this tank, removing them from the main display. Proponents of the system claim that it is so efficient at waste removal there is no need for protein skimming. The filter tank has 24hr. lighting to encourage the growth of the macro-algae which is harvested regularly to remove the processed wastes from the system. The sump also allows the growth of many micro organisms, like plankton and small invertebrates which are returned to the main tank as fresh, live food.
Marine Fish and Invertebrate Systems Keeping : Berlin System : Tanks : Equipment Dreams : Marine Mud : Jaubert System :