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 . <EOS>