An artificial ecosystem is a man-made environment that is designed to mimic a natural ecosystem. It allows for the study and observation of various species in a controlled setting.

Option 1, a pisciculture tank, refers to a tank or pond specifically used for fish farming. While it may contain various fish species, it is not a complete ecosystem as it lacks the presence of other organisms such as plants and predators.

Option 2, agricultural land, refers to farmland that is used for cultivating crops. While it may support some organisms, it is primarily focused on crop production and lacks the diversity and complexity of a natural ecosystem.

Option 3, a zoo, is a facility where animals are kept in captivity for public viewing. While it may have different species coexisting, it is not a self-sustaining ecosystem as the natural food chains and interactions are significantly altered.

Option 4, an aquarium, is a glass tank or container that houses aquatic organisms such as fish, plants, and invertebrates. It provides a controlled environment but lacks the complexity and interconnections of a larger ecosystem like a river or ocean.

In conclusion, the most suitable representation of an artificial ecosystem among the given options is option 4, the aquarium.