Why do you need to Supply Oxygen to a Tropical Tank when Fish are capable of Surviving Without extra oxygen in Lakes and Oceans?

The expert we contacted on this question told us this: "You have to provide oxygen to any tank that has more fish (thus higher oxygen demand) than the plants and algae in that atmosphere can supply".
Most natural bodies of water are alive with oxygen-producing plants, the plant life in a home tank is like a sprig of parsley next to a piece of halibut in a restaurant.
http://wikipedia.org/wiki/Fish_tanks#Design