A reagent injector and/or aerator for aqueous fluids can use a variety of different removable end caps to adapt a venturi to different types of water treatment systems. The invention is adjustable because the end caps can be removed and springs that activate poppet valves can be changed to accommodate changes in the pressure or flow requirements of the system in which it is used. In addition to, or instead of, atmospheric air, reagent chemical solutions or gasses can be entrained into a flowing water stream by the venturi.