Disclosed is an apparatus for housing a block for chemically dosing a stream of fluid flowing past at least one surface of the block, by dissolution and/or erosion of the block, the block consisting of one or more active ingredients and a solid carrier of polyethylene glycol for carrying the one or more active ingredients, the apparatus including a chamber for the block to reside in, the chamber having an inlet and an outlet to enable the stream of fluid to flow there through, wherein the chamber is shaped to enable the block to reside therein with opposing side walls of the block abutting the side walls of the chamber such that, in use, the at least one surface of the block is dissolved and/or eroded without a substantial change in the area of the at least one surface thereby providing a constant rate of chemical dosing to the stream of fluid for a given flow rate.
