There is disclosed a cleaning apparatus for a radiation source assembly in a fluid treatment system. The clean apparatus comprises a cleaning sleeve moveable to remove fouling materials from an exterior portion of the radiation source assembly, the cleaning sleeve comprising at least one chamber for receiving a cleaning fluid and a cleaning sleeve inlet in fluid communication with the at least one chamber and a first conduit element for conveying the cleaning fluid to the cleaning sleeve inlet, the first conduit element being configured such that a distal portion of the first conduit element is in fluid communication with the cleaning sleeve inlet and a proximal portion of the first conduit element is disposed outside of fluid being treated in the fluid treatment system.