The big benefit of an air-stone is that it moves the water from the bottom of the tank to the surface, which your average internal filter may not do very well. The surface of the tank is where the gas-exchange of oxygen and CO2 happens... The actual oxygenation by bubbling air through the water is miniam, but by shifting the water at the bottom up to the top, you can improve the oxygen levels quit a bit.

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Mats