The ornamental design for an ornament for aquarium, substantially as shown and described.
FIG. 1 is a perspective view from the front and to one side of an ornament for aquarium incorporating the design;
FIG. 2 is an elevation view of both the front and the rear thereof, as they are identical;
FIG. 3 is an elevation view of both sides thereof, as they are identical;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a perspective view from the front and to one side thereof as shown in a collection thereof.
The dotted and/or broken lines depict environmental features, or the character or contour of the surfaces represented and are not a part of the claimed design.