Absent of any visible equipment an aquarium can be viewed as a natural work of art.
The necessary life support system can be hidden in the base of the aquarium.
Nothing is more challenging nor more informative than the natural processes of life in closed water aquariums systems.
To a degree an aquarium can be self supportive or "balanced".
The Smithsonian Institute has built "macro" environmental models of real biotopes such as marsh wetlands and tidal pools which are self sustaining as in nature.