The ornamental design for a reservoir for activeingredient, as shown and described in the attached drawings.
FIG. 1 is a perspective view of the design.
FIG. 2 is a front elevation view of the design.
FIG. 3 is a rear elevation view of the design which is identical to the front elevation view of the design.
FIG. 4 is a left side elevation view of the design.
FIG. 5 is a right side elevation view of the design which is substantially identical to FIG. 4.
FIG. 6 is a top view of the design; and,
FIG. 7 is a bottom view of the design.
Elements represented by broken lines and shade lines depicting environmental structure are for illustrative purposes only and form no part of the claimed design.