To identify the statements which are not true for the natural ecosystem, let's analyze each statement:
(A) Grassland and desert are examples of terrestrial ecosystems. This statement is true as both grasslands and deserts are natural terrestrial ecosystems.
(B) Oceans and rivers are man-made ecosystems. This statement is false. Oceans and rivers are natural aquatic ecosystems, not man-made.
(C) Wetland and estuary are examples of aquatic ecosystems. This statement is true as both wetlands and estuaries are natural aquatic ecosystems.
(D) Crop fields and aquariums are natural ecosystems. This statement is false. Crop fields and aquariums are man-made or artificial ecosystems.
Thus, the statements that are not true for natural ecosystems are (B) and (D). Therefore, the correct answer is (B) and (D) only.