Abstract: An apparatus for efficiently circulating and moving a fluid across a workpiece. The apparatus can provide for the automated handling of the fluids used, and is well suited for use in the staining and fixing of biological assays such as electrophoresis gels. The tray of the invention includes first and second side walls, first and second end walls, a first curved wall portion connecting the first side wall and the first end wall, and a second curved wall portion connecting the second side wall and the second end wall. The inclusion of the curved surface in the combination causes mixing to occur efficiently when the tray is "rocked" from side to side as in the conventional apparatus. Thus, the apparatus of the invention may also include a motor for alternately raising and lowering each end wall of the tray in a rocking motion, thereby causing the fluid to alternately flow towards and away from each end wall.