The short answer
For most tanks, an air pump is optional, not necessary. If your filter already ripples the water surface, your tank is getting enough oxygen. An air pump becomes necessary in specific cases: running a sponge filter, keeping a warm or heavily stocked tank, or any low-oxygen situation. So the honest answer is: usually a nice-to-have, occasionally essential.
How oxygen actually gets into the water
Fish don’t breathe the bubbles — they breathe dissolved oxygen. Oxygen enters the water at the surface, and the more the surface moves, the more gas exchange happens. That’s why a filter with a decent outflow, or a spray bar breaking the surface, oxygenates a tank perfectly well on its own.
An air pump helps because rising bubbles agitate the surface. The bubbles are visible reassurance; the surface movement does the real work.
When you genuinely need one
- Sponge filters: these are air-driven, so a pump isn’t optional — it powers the filter.
- Heavily stocked tanks: more fish means more oxygen demand.
- Warm water: high temperatures (or medications that reduce oxygen) call for extra aeration.
- Poor surface movement: if your filter barely ripples the top, a pump helps.
Watch your fish. If they hang near the surface gasping in the morning, that’s a low-oxygen signal and an air pump will help immediately.
When you can skip it
- A well-filtered tank with visible surface ripple.
- Labyrinth fish like bettas and gouramis, which gulp air directly.
- Planted tanks running CO2 — here extra aeration can actually drive off the CO2 you’re adding.
If you do want one, see the air pumps hub and our best aquarium air pump picks. Running a sponge filter? Check the internal filter guide too, and read do I need a filter.