The ornamental design for a tankbase, as shown and described.
FIG. 1 is an exploded top plan view of a tank base showing our design, an exploded bottom plan view therein being a mirror image of the exploded top plan view;
FIG. 2 is a top plan view of the tank base of FIG. 1, the bottom plan view being a mirror image of the top plan view;
FIG. 3 is an exploded top perspective view of the tank base of FIG. 1;
FIG. 4 is an exploded front facing side view from a perspective of a center point of the exploded tank base of FIG. 1, an exploded rear facing side view from the center point of the exploded tank base being a mirror image thereof;
FIG. 5 is an exploded front facing side view of the tank base of FIG. 1, an exploded rear facing side view of the tank base being a mirror image thereof; and,
FIG. 6 is an in-use front facing side view of the tank base of FIG. 1 having a tank thereon.
The broken line showing of the tank in FIG. 6 is for the purpose of illustrating environmental structure and forms no part of the claimed design.