An adjustable Venturi is disclosed, and having a plurality of inlet ports providing substances to be mixed with a flow of fluid through the Venturi. In one embodiment, the Venturi is constructed having an inlet portion, an outlet portion, with an annular mixing chamber coupled to inlet ports so that substances provided through inlet ports are mixed in the mixing chamber, and such mixture introduced into the flow. A gasket sealably disposed between inlet portion and outlet portion is selectable in thickness, for adjusting width of a Venturi interface between the inlet portion and outlet portion. In another embodiment, the inlet portion and outlet portion are fitted in an elongated opening, and are replaceable to change operational characteristics of the Venturi. A replaceable disk having a central opening and substance-providing passageways is disposed between the inlet portion and outlet portion, and may be changed to change characteristics of the Venturi.