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The oxygen in the water will, at least in part be consumed by the fish and some by microorganisms.It will be supplied by plants and by diffusion.On the whole the rate of diffusion of oxygen into the water will not be more than the requirements of the fish and other occupants of the bowl- it will be exactly the same.To a rough approximation, the rate of oxygen delivery will be the same as that which would be required to burn the fish food.That might be the easiest thing to estimate.
Where did you see the plants?
In any thriving fishtank, I also see a stream of bubbles floating from the bottom to the top, driven by a pump.Fine bubbles greatly increase the surface area in contact between the air and water, increasing oxygen delivery and carbon dioxide extraction.
There are bowls without oxygen pumps.