A guide plate 45 is placed at a guiding and holding member 42 which is fixed to a region of an inner panel 34 adjacent to the upper end portion 49A of a B pillar garnish. Living organisms are added into a vessel’s ballast water tank to consume oxygen dissolved in ballast water contained in the ballast water tank, and food sources for selected organisms contained in the ballast water. The fluid cassette is retained within the device on a base plate partially disposed within a device housing interior, while a housing cover is selectively opened and closed to permit access to the base plate. The reactor provides high activity, low pressure drop, and high heat transfer.