1/4" John Guest Inline Check Valve will keep water moving one direction.  If you use a pressurized bladder tank to store RO drinking water, this check valve will prevent the RO water from surging through the DI resin stage.

Cut tubing and insert firmly into both ends.  Arrow on fitting indicates that water flows to the bladder tank.  If installed backwards, no RO water will be collected in the bladder tank.