A removable cap assembly employs a cap with a curved side wall having an inner surface and an outer surface. A top portion extends from the curved side wall. A boss extends perpendicularly from the top portion. The boss further has a bottom and a top with a circumferential rib extending perpendicularly from the top and at least one ridge on the inside surface of the boss below the top and above the bottom. A stopper is dimensioned to seal the opening. The stopper has a hole therein configured to allow a fluid to flow therethrough. The stopper is maintained in the cap by the boss and the at least one ridge. The stopper includes a disc shaped base and a cylindrical wall portion extending from the base having at least one cavity corresponding to the location of the at least one ridge on the boss. A first groove in the base surrounding the wall portion is dimensioned to receive the bottom of the boss.