KOK Filter SystemKOK Series Filtration Combo provides mechanical and biological filtration at a single unit which complete with bobbin wound filter, 6 way multiport valve, pipings, circulation pump, air blower, control box and base. It has been engineered uniquely for pond and water garden. Description About Biological filtration: Biological filtration is the essential part of the Nitrogen Cycle. It is an effective way to remove ammonia and break down into nitrites.