Which of the following could not be considered an ecosystem?

Ecosystem consists of all the living organisms in the biological community and the environment they live in i.e. the area where the group of living organism interacts with the non living organism is termed as an ecosystem. Just the fishes would not make an ecosystem. Fishes along with aquatic environment they live in makes an ecosystem.

Coral reef ecosystem: includes small coelentrates living in the marine water and are held together by their  calcium carbonate structure. 

Ponds fall under the category of freshwater ecosystem.  It contains phytoplanktons, zooplanktons and other organisms. Hence, a small and is an ecosystem because their is an interaction of the living community with their environment.

A pile of dung is an ecosystem because it will have certain living organisms interacting with each other, depending on each other for nutrients.