Abstract: A fluid treatment or monitoring assembly has a number of elongate, parallel elements (7) such as filters mounted in a closed vessel (1). Each element (7) is held in place by an individual spring-loaded locator (15) bearing down on one end, holding its other end against a seat. The locators are mounted on a common plate (16) which is secured by screw rods (12) parallel to the filters. Each rod (12) carries a projection (20) and as it is rotated the projection engages any locator (15) that is not properly retaining a filter element (7), thus preventing completion of the assembly. But if all the filter elements are present and properly located, the rods (12) can be fully screwed down. In addition, the vessel cover (2) may be impossible to fit unless all the rods (12) are screwed down.