FIG. 1 is a perspective view of a water tank, showing our new design; FIG. 2 is a front elevation view thereof; FIG. 3 is a rear elevation view thereof; FIG. 4 is a side elevation view thereof; FIG. 5 is a another side elevation view thereof; FIG. 6 is a top plan view thereof; and, FIG. 7 is a bottom plan view thereof.