The council shall cause the list of nominees named by it to be published at least once, which publication shall be not more than fifty days nor less than thirty days prior to the date fixed for the election. The publication of notice shall also contain a statement that additional nominees for council members may be made by petition of twenty-five or more qualified voters residing within the district filed with the council within twenty days after the publication of notice of election. The names of all such nominees shall be placed upon a ballot to be submitted to the voters of the respective districts at the election.