A tank for a water-heating appliance including a body, a first head, and a second head. The body includes a first end, a second end, and a longitudinal axis. The body extends along the longitudinal axis from the first end to the second end. The first head includes multiple landings. The first head is coupled to the first end so that the first head is convex and extends away from the body. The longitudinally outermost points of the landings define a first plane perpendicular to the longitudinal axis. The second head includes multiple landings. The second head is identical to the first head except for at least one opening through a landing. The second head is coupled to the second end so that the second head is convex and extends away from the body. The longitudinally outermost points of the landings define a second plane perpendicular to the longitudinal axis.