The filter has an inlet (left side of filter) and outlet (right side) along with the injection tube on the head. A small pump sits inside the return side of my wet/dry and pumps water to the inlet (left side) and throught the filter. The filtered water is then returned to the inlet side (flow adjustment valve) of the wet/dry then back into the tank. This is the only biological filter system used on my tank at this point unless you consider the live rock.