Abstract: Apparatus for mixing fluids at a fixed ratio. It includes a large tank for the mixture, a storage tank for an additive fluid and preferably, an intermediate tank having a volume at least equal to the amount needed to provide the desired ratio when the large tank is completely filled with the larger volume of fluid and the additive from the intermediate tank. A stand pipe in the large tank and pipes and valves between the tanks are arranged to enable the introduction of the main fluid into the large tank, through a fill valve, to simultaneously displace a proportional amount of additive fluid into the large tank by the action of the main fluid rising in the stand pipe. An interlock arrangement between the fill valve and the other valves assures proper refilling of the large tank with the correct mixture.