A system for dosing a stream of fluid with at least one active substance is disclosed. The system comprises a housing which defines a chamber having an inlet (13) and an outlet (14) to enable the flow of fluid through the chamber. A block containing the active substance is placed in the chamber that has a body with an outer surface and a passage there through defining an inner surface. The block is positioned within the housing chamber in an operative position between the inlet and the outlet so that in use fluid flows over the inner and outer surfaces of the block to dissolve and/or erode the block. the block consists of a solid carrier for carrying the active substance(s). The active substance(s) are evenly distributed throughout the solid carrier matrix. The active substance(s) in the block may consist of surfactants for fire fighting applications, detergents for cleaning applications or fertilisers, pesticides, insecticides or herbicides for agricultural or gardening applications or any other one or more active ingredients as desired. The one or more active substances may also comprise a foam stabiliser(s) and may also comprise a corrosion inhibitor(s). A suitable solid carrier is polyethylene glycol having an average molecular weight of between 1000 and 8000MW.
