The ornamental design for a septictankscreen, as shown and described.
FIG. 1 depicts a front perspective view of the septic tank screen of the present design application where a septic tank opening is depicted in broken lines to show context and forms no part of the design to be patented;
FIG. 2 depicts a top plan view of the ornamental design shown in FIG. 1;
FIG. 3 depicts a bottom plan view of the ornamental design shown in FIG. 1;
FIG. 4 depicts a side elevation view of the ornamental design shown in FIG. 1, wherein the septic tank screen has left and right symmetry; and,
FIG. 5 depicts a front elevation view of the ornamental design shown in FIG. 1, wherein the septic tank screen has front and rear symmetry.