A flow pipe and one or more permanent magnets placed in or on the flow pipe, each permanent magnet presenting at its axial ends an infinite divergence of the magnetic field it generates. Fluid is treated by flowing through a flow pipe equipped with at least one such permanent magnet. The permanent magnet can be made of a plastic coextrudate with a ferromagnetic material such as iron powder, iron dust or iron chips. The permanent magnets can be arranged in groups with flow turbulence initiators positioned within the flow pipe between the magnet groups.