A water filter connects into a waterline which feeds a faucet separate from the filter. A filter cartridge is seated within a canister and has an inlet and an outlet. The canister is removably attached to a cap in which is disposed an entrance port to communicate with the outlet and an exit port to communicate with the inlet. Also in the cap are an input port for receiving water from the line, an output port for delivering water into the line to feed the faucet, a passage which communicates between the input port and the exit port, a passageway which communicates between the input port and the output port and a channel which communicates between the entrance port and the output port. A diverter in the cap responds to a rate of water flow from the faucet of less than a predetermined amount for opening the passage and the channel and closing the passageway, while being responsive to a rate of flow greater than such amount for closing the passage and the channel but opening the passageway. A selector at the faucet is manually adjustable to determine which rate of flow occurs, that selection making the determination of whether the water delivered to the faucet has been filtered or has bypassed the filter. Alternative coupling arrangements permit the water filter to be readily connected into an existing water line of essentially any orientation by cutting that line at only one point.