Abstract: An aquarium filtration system, biological and mechanical in nature, having the capability to include chemical filtration. The system uses high-density biological filter elements in a series of chambers operated by a single pump. Both aerobic and anaerobic bacteria colonies are used for water filtering.

Abstract: A writing instrument having two writing elements is described. A tubular cam is utilized together with cam followers and element holders for each writing element and associated supporting structure to provide an instrument which will position one element in the extended position while positioning the second element in the retracted position without the use of springs, will lock each element in its respective position and which will be simple to assemble and maintain.