The common assumption is that bubbles from an airstone directly oxygenate the water, but the actual mechanism is simpler: rising bubbles agitate the surface, and that surface disturbance is what drives gas exchange, letting oxygen in and CO2 and other dissolved gases out. A filter return that already breaks the surface can accomplish the same thing without a separate air pump.
Air pumps become genuinely necessary in a few specific situations: heavily stocked tanks where oxygen demand is high, tanks running during a power outage on battery backup where the main filter has stopped, hot summer conditions where warmer water holds less dissolved oxygen, and any tank using certain medications that reduce oxygen levels as a side effect.
The tank that needs an air pump the most is the one you'll forget about it for — during a heat wave, or when the filter clogs and nobody's home.
In a CO2-injected planted tank, however, an air pump running around the clock works against you, stripping out the CO2 you're paying to inject. If you run one at all in this setup, it should switch on only during the lights-off period, after CO2 injection has stopped for the day.
Standard, moderately stocked community tanks with a filter that already agitates the surface generally don't need a dedicated air pump. Heavily stocked tanks, hot climates without air conditioning, and backup preparedness for power outages are the situations where the modest cost is genuinely worth it.
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