The device is for a container provided with an opening and a flexible wall. The device comprises at least one tank insertable into the container in operating position; and at least one release mechanism insertable into the container in operating position. The release mechanism is capable of cooperating with the tank in order to connect the tank with the interior of the container in response to a pressure exerted on the release mechanism via the wall of the container. The device comprises also a support element insertable inside the container to support in operating position the tank and the release mechanism in relation to the container. The support element has a fastening to fix it on the container close to the opening.