Biological filters are positioned to create the beginning of the waterfall in your pond. The filter uses bacteria to break down pond wastes, converting them into less harmful compounds that can be used as aquatic plant fertilizers. Water enters the biological filter from the skimmer via flexible pipe located near the base of the unit. The water then flows from the bottom to the top of the filter, traveling through filter media housed inside the unit. The filter media helps with the removal of fine to medium-sized particles.