A fluid filter comprises a filter housing, having an inlet end (202) and an outlet end (204), and a plurality of bundles of fibres (211) extending longitudinally of the housing and being secured at the inlet end. The fibres are charged with an electric or magnetic field to selectively block or allow the passage of particles of a known charge present in the fluid through to the outlet end. To flush the filtered particles out of the filter, the charge imparted to the fibres is reversible to allow the charged particles to flow freely through the filter. The filter can also be used to control the settling characteristics of particles in a fluid.