An ink tank and ink tank support structure which contains structural elements which are designed to permit proper installation of the ink tank in the ink tank support structure while minimizing the chances of improper installation of the ink tank in the ink tank structure. A primary ink tank seal is provided in the ink tank, and a relatively low compressive force seal is provided between the ink tank support structure and the ink tank to reduce leakage of fluid from the tank and limit evaporation of fluid from the tank despite repeated insertions and removals of the ink tank from the ink tank support structure. A variable capacity ink tank and an ink tank with staggered height walls are disclosed, as well as a code reader for determining characteristics of ink tanks.