Question

why is a pond self sustaining unit while an aquarium may not be

Answer 1

Since a pond is a natural ecosystem, it has all the biotic and abiotic components in it and these components are interlinked with eachother for ensuring the survival of the organisms there, hence its a self sustaining unit. Where as, an aquarium is an artifitial ecosystem, it is made by man. So, if any fish or other organisms present in the aquarium die, its dead body will not be acted upon by decomposers cause of its absence. Also there is no sunlight and other abiotic factors, so the survival of the organisms in the aquarium is to be relied upon man who has to ensure the maintenance of the aquarium , so the aquarium is not a self-sustaining unit.