A fluid mixing device has an inlet passageway for connection to a primary fluid supply, an outlet passageway; and at least two separate venturi passageways each connected between the inlet and outlet passageways. Each venturi passageway has a throat portion, a first venturi inlet connected to the throat portion of the first venturi passageway and a second venturi inlet connected to the throat portion of the second venturi passageway. The first and second venturi inlets are connected to supplies of two different secondary fluids for mixing into the primary fluid, a first one of the secondary fluids being of lower viscosity than the second. The outlet passageway is spaced a predetermined distance downstream of the venturi inlets to prevent substantial backflow of the first secondary fluid mixture into the second venturi passageway prior to mixing together of the two streams in the outlet passageway.