Councilmen shall be qualified electors of the city and, in the case of candidates for any district, they shall have been residents of the district which they represent for at least 90 days prior to their election, shall reside in the district during their terms of office, and shall have attained the age of 21 years. No councilman shall hold any other public office except that of notary public or member of the National Guard or naval or military reserve, and all shall have been residents of the city for at least one year prior to their election. If the councilman shall cease to possess any of these qualifications or shall be convicted of crime involving moral turpitude, his office shall immediately become vacant.