The ornamental design for a tankcover, as shown and described.
FIG. 1 is a perspective view of the tank cover.
FIG. 2 is a front elevational view of the tank cover.
FIG. 3 is a rear elevational view thereof.
FIG. 4 is a top plan view thereof.
FIG. 5 is a bottom plan view thereof.
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a right side elevational view thereof.
The broken lines showing in FIGS. 1 and 2 are for the purpose of illustrating the environment of the article only and form no part of the claimed design.