The ornamental design for a double-walled tank, as shown and described.
FIG. 1 is a top and front and left side perspective view of the double-walled tank showing our new design;
FIG. 2 is top plan view thereof;
FIG. 3 is a front view thereof the rear view being a mirror image thereof; and,
FIG. 4 is a left side elevational view thereof the right side being a mirror image thereof.
The broken lines shown in the figures are for illustrative purposes only, and the subject matter depicted thereby constitutes no part of the claimed design.