We claim the ornamental design for a toilet tank, as shown and described.
FIG. 1 is a top perspective view of the claimed design;
FIG. 2 is a front elevation view of the claimed design of FIG. 1;
FIG. 3 is a left-side elevation view of the claimed design of FIG. 1;
FIG. 4 is a right-side elevation view of the claimed design of FIG. 1;
FIG. 5 is a rear elevation view of the claimed design of FIG. 1;
FIG. 6 is a top plan view of the claimed design of FIG. 1; and,
FIG. 7 is a bottom plan view of the claimed design of FIG. 1.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design.