The invention relates to an easy-to-install, mechanically-operated automatic/manual device for controlling an outlet for water or any fluid. The inventive device comprises a threaded inlet nozzle which can be used for water or any fluid and which can be pivoted in order to direct said device as appropriate. The device also contains two reverse polarity magnets, a spring, two oilcloth membranes with openings, one oilcloth membrane with no openings, two supports for said membranes, an air chamber, a metal ring that can be actuated at will, an operating mechanism having on-off positions, a copper double tube which is submerged in the container to be filled, an oilcloth tube that connects the copper double tube to the air chamber, an operating mechanism having manual and automatic positions that is used to operate said device. The invention also relates to the operation of the device, the purpose of which is to close off the water or fluid passage when the receptacle, cistern, basin, tub, trough, bath, toilet, swimming pool or other container is full using hydrostatic pressure which is exerted by the container of water by means of the copper double tube which is connected to the air chamber by the oilcloth tube. Said air chamber exerts the pressure necessary to separate the magnets and, therefore, to close the fluid passage opening.