The ornamental design for a dirtsucker for an aquarium, as shown and described.
FIG. 1 is a perspective view of a dirt sucker for an aquarium showing my design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof; and,
FIG. 6 is a bottom plan view thereof.
The intermittent lines contained within the circular area and completely contained within solid lines towards the lower end of the claimed design in FIG. 1 are included for the purpose of illustrating portions of the dirt sucker for an aquarium that form no part of the claimed design.