A biological filter includes the foam filter material in addition to biological elements(bioballs) The elements provide a large surface area for beneficial bacteria to grow. This bacteria will take a few weeks to develop on the bio balls once the filter and pump system are placed in the pond. Acting in a natural way to balance the pond environment while consuming nitrates that are created by fish waste and decomposing organic materials. These nitrates left unchecked feed the algae and result in great algae blooms(green water) All TotalPond filters are also fish safe and energy efficient.