A floating separating element for use in centrifugal separation of components of a physiological fluid comprises a positioning part and a separating part, where the positioning part is designed to automatically assume a position in a supernatant and a separating part is positioned at a desired location with respect to the interface between the supernatant and heavier components. In preferred embodiments the physiological fluids are blood or bone marrow aspirate, and the heavier components comprise red blood cells. The positioning part comprises the majority of the mass of the separating element and is thin so that differences in the position of the separating element with respect to the interface are small compared to differences in the densities of the separated compoments, particularly the component comprising red blood cells. A method allows red blood cells to move the separating element during decanting to ensure complete decant of the supernatant.