The ornamental design for an aquarium covering, as shown and described.
FIG. 1 is a front elevational view of an aquarium covering showing my new design;
FIG. 2 is a back elevational view thereof;
FIG. 3 is a right elevational view thereof;
FIG. 4 is a bottom elevational view thereof;
FIG. 5 is a top elevational view thereof; and,
FIG. 6 is an exploded isometric elevational view thereof.
The even spaced broken lines and dash-dot-dot-dash broken lines in FIG. 6 are included for the purpose of illustrating portions of the aquarium covering and form no part of the claimed design.