This is one way to of introducing air (Oxygen) into the fish tank or pond water, and at the same time by agitating surface skin of the water by the movement of the rising air bubbles, allows the exchange of gases. This is similar shaking (or agitating) a bottle of pop where the carbon dioxide is released into the environment (i.e. Co2 is removed back into the air and Oxygen (O2) dissolved into the water).
So to summarise: Oxygen goes from the air to the water, and carbon dioxide goes from the water to the air.
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