The disclosure includes devices and methods for controlling fluid flow. The devices include a tube configured to receive fluid from a fluid source; and a valve for controlling fluid flow through the tube, the valve responsive to fluid pressure in the tube. The valve includes a first magnet outside the tube at a fixed first distance from the tube and a second magnet outside the tube substantially opposite the first magnet and at a second distance from the tube, the second distance greater than the first distance. The valve includes a ferromagnetic fluid within the tube between the first magnet and the second magnet and held in position by a magnetic field generated by the first magnet. The amount of ferromagnetic fluid is sufficient to seal the tube when the fluid pressure is below a threshold pressure set by the second distance between the second magnet and the tube.