I claim the ornamental design for a water collector tank, as shown and described.
FIG. 1 is a front elevation view of the water collector tank showing my novel design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a perspective view thereof, with the lid to the inner compartment open;
FIG. 8 is a perspective view thereof; and,
FIG. 9 is a left view thereof, including an unclaimed illustration of a typical environment of surrounding soil in which the water collector tank may be partially buried.
All of the broken line showing is included for the purpose of illustrating and forms no part of the claimed design.