Passive control of fluid flow from a vessel is provided by a floating weir assembly. The assembly includes a float and a weir opening that is in fluid communication with an outlet from the vessel. The weir opening and the fluid path connecting the weir opening and the outlet are designed such that the rate of outflow is determined by the geometry and position of the weir opening. The weir opening is preferably vertically adjustable so that desired outflow rates can be specified.