Why you must give oxygen to fish aquarium?

Since there is not enough oxygen in the glass aquarium water at home, it is necessary to give oxygen to the water with the help of a machine. The bubbles that are seen all the time in the water in the glass jar are actually the outside air, which is inserted under the water so that the required oxygen from there can mix with the water.

Then why the pond does not need oxygen? How do fish live there? The pond may also need oxygen if there are large numbers of fish.

This is because the water in the pond is saturated with oxygen released from the algae. But there are almost no algae in the fish jar water. So special arrangements have to be made to give oxygen.