1/4" fpt x 1/4" O.D. tubing John Guest RO Tank Shut Off Elbow ( JBV500822W ), with quick connect for Microline and many older models using 1/4" tubing to the Tank.
Can be used on all under the sink RO holding storage tanks ( since they all have 1/4" mpt connections ).
NOTE: There is also a larger version for 3/8" O.D. tubing, JBV501222W.