Abstract: A liquid reservoir, for example a swimming bath, a test tank or an aquaculture tank, comprises an open-topped container having at least one side wall, a first floor fixed relative to the side wall(s) and a second floor above the first floor which is able to move upwardly and downwardly relative to the side wall(s) in order to vary the height and/or inclination of the second floor in relation to the first floor. The reservoir comprises means for varying the specific gravity of the second floor between a value above and a value below the specific gravity of the liquid of the reservoir. Passageways between the side edge of the second floor and the side wall(s) of the container and/or through the second floor enable liquid to flow between the space between the two floors and another space above the second floor when the height of the second floor is varied.