Abstract: A clamp-on magnetic water treatment device for minimizing hard precipitate scale and lime deposit in a fluid supply, especially a domestic water system, has a magnet array having two sections which clamp together over a non-ferromagnetic section of conduit in the water system. The array of magnets defines at least two pairs of magnetic poles of opposite polarity across the flow path for the water, whereby a magnetic field is produced defining flux lines directly perpendicular to the flow path, and the magnets in the array attract one another across the flow path. The magnets in the array also define magnetic poles of opposite polarity proceeding downstream along the flow path. However, the individual magnets in the array are all arranged such that their poles are directed toward and away from the conduit, and not longitudinally along the flow path. In this manner, the maximum field strength is obtained in a clamp-on device.