A method and apparatus produces an ionic change in fluid by causing the fluid to flow in specified sequence over electrodes having different electrochemical potentials. The last electrode which the fluid contacts is so selected as to provide the desired ionic change. The electrodes (1, 2) are provided in a tube of electrically insulative material (3) such as plastic. At least some of the electrically conductive material of one of the electrodes is disposed downstream of all of the electrically conductive material of the other of the electrodes, so that fluid flowing axially through the tube sequentially contacts the electrodes and is only acted on by the downstream electrode before flowing from the tube. By appropriately selecting the material of the downstream electrode, decoloration of fluid having metal oxides or a REDOX reaction can be carried out.