A method and apparatus for treating a fluid to destroy, remove, or reduce undesirable agents, such as microorganisms, particles or ions, contained in the fluid and/or to inhibit the formation of scale are disclosed.The invention includes an apparatus for treating a fluid to destroy, remove, or reduce undesirable agents, such as microorganisms, particles, or ions, contained in the fluid and/or to inhibit the formation of scale. The apparatus can include an open fluid directional means or conduit. The preferred open fluid directional means is a conduit manufactured of resilient, corrosion-resistant material. The apparatus is attached to a power source and has a controller means. The controller means is desirably a microprocessor and can include a switching means for regulating the current to the apparatus. However, simpler circuits and components can be used for the controller means. The apparatus includes a field generating means. The field generating means forms a treatment field. The treatment field is a magnetic field and/or an electrical field in an effective dose sufficient to treat the fluid. The effective dose is determined by the exposure time(s) and strength(s) of the treatment field(s) and the concentration of contaminants present in the fluid subjected to the treatment.