A coolant filter for use in filtering a coolant solution which flows through the coolant filter includes a filter housing assembly which is made up of an outer housing (21) which is crimped to a nutplate (22) which defines an internally threaded flow outlet. A generally cylindrical filter element (23) is positioned inside of the filter housing assembly and a first endplate (24) is bonded to the end of the filter element which is adjacent to the nutplate. An endplate member (46) is provided for attachment to the opposite end of the filter element and is configured in such a way so as to define an interior chamber where supplemental coolant additive pellets are stored. A closing plate (25) is applied across the open end of the interior chamber so as to create an enclosed chamber for the supplemental coolant additive pellets (39). A slow release mechanism is provided for controlling the rate of release of the supplemental coolant additive from the enclosed chamber into the coolant solution. In one embodiment of the present invention, the slow release mechanism includes a diffusion tube (56) which defines a diffusion orifice (57). In another embodiment of the present invention, the slow release mechanism includes a semipermeable (or osmotic) membrane wafer. In both instances, the slow release mechanism is positioned between the source of supplemental coolant additive and the flow outlet in the nutplate.