A method and an apparatus for treating a fluid are disclosed. The apparatus includes a cylindrical chamber of non-magnetic material for holding a volume of fluid to be treated. The fluid contains a quantity of magnetic particles, preferably nanoparticles, having desired properties for treating the fluid. The apparatus includes a magnetic field generator for creating a non-static magnetic field within the chamber, thereby to induce motion in the magnetic particles within the chamber in use. The chamber has an inlet through which fluid to be treated can be introduced, and an outlet through which treated fluid can be removed from the chamber. Sets of windings are disposed concentrically about the chamber and arranged to create a rotating magnetic field within the chamber. Preferably the rotating magnetic field rotates in the opposite sense to swirling rotation of the fluid in the chamber. This enhances contact between the nano-particles and the fluid to be treated.