According to the Rodex, a speices that studies bioligy, Aadak's toa stone was altered. If there was one change in it's color, the element could change with it. Therefore, Aadak's element was nitrogen.
His life as a Toa did not last long though. when he fufilled his destiny and chose to become a turaga, Aadak's element refused to cooperate, so it turned on him and killed him. Takuru's tales mention him, so he cannot be forgotten.