Abstract: A method of treating fluids to induce coagulation of particles comprises subjecting the fluid to the influence of a repulsive magnetic field, that is a magnetic field generated between opposed like magnetic poles as distinct from a magnetic field generated between North and South poles. Preferably oligo-dynamic particles of mineral or other substances are introduced into the fluid before, during or after magnetic treatment. The preferred method of particle introduction is by sacrificial anode incorporated in an electrical circuit with a cathode and the fluid as an electrolyte. The invention may be applied to the treatment of hard water to prevent or reduce deposition of scale and in the treatment of swimming pool water to eliminate bacteria and/or control formation of algae. A variety of other uses in relation to both liquids and gases are described.