An automatic aeration device comprising: • a first floating roof, a second floating roof and a movable valve; • wherein the first floating roof has a through hole which has a periphery provided with a first enclosure extending upward vertically through a first height; • the first enclosure has an upper end provided with a first upper flange; • the second floating roof has a maximum area smaller than an area of the through hole of the first floating roof; • the second floating roof has an upper face provided with a second enclosure extending upward vertically through a second height that is greater than the first height of the first enclosure; • the second enclosure has an upper end provided with a second upper flange; characterized in that • the movable valve has a plate provided with a central hole which extends vertically through the plate; • the plate has a lower portion provided with an outer wall and an inner wall arranged concentrically; • the outer wall and the inner wall extend downward vertically through a determined depth; • the outer wall has an outer diameter smaller than an inner diameter of the first enclosure and has a lower end provided with a first lower flange interfering with the first upper flange; • the inner wall has an outer diameter smaller than an inner diameter of the second enclosure and has a lower end provided with a second lower flange interfering with the second upper flange; • the outer wall has an upper end provided with a plurality of first vent holes; • the inner wall has an upper end provided with a plurality of second vent holes; and • the movable valve is mounted on and covers an upper portion of the through hole of the first floating roof, with the outer wall being mounted in the first enclosure of the first floating roof, and with the inner wall being mounted in the second enclosure of the second floating roof. • a first floating roof, a second floating roof and a movable valve; • wherein the first floating roof has a through hole which has a periphery provided with a first enclosure extending upward vertically through a first height; • the first enclosure has an upper end provided with a first upper flange; • the second floating roof has a maximum area smaller than an area of the through hole of the first floating roof; • the second floating roof has an upper face provided with a second enclosure extending upward vertically through a second height that is greater than the first height of the first enclosure; • the second enclosure has an upper end provided with a second upper flange; characterized in that • the movable valve has a plate provided with a central hole which extends vertically through the plate; • the plate has a lower portion provided with an outer wall and an inner wall arranged concentrically; • the outer wall and the inner wall extend downward vertically through a determined depth; • the outer wall has an outer diameter smaller than an inner diameter of the first enclosure and has a lower end provided with a first lower flange interfering with the first upper flange; • the inner wall has an outer diameter smaller than an inner diameter of the second enclosure and has a lower end provided with a second lower flange interfering with the second upper flange; • the outer wall has an upper end provided with a plurality of first vent holes; • the inner wall has an upper end provided with a plurality of second vent holes; and • the movable valve is mounted on and covers an upper portion of the through hole of the first floating roof, with the outer wall being mounted in the first enclosure of the first floating roof, and with the inner wall being mounted in the second enclosure of the second floating roof.