The key question is whether the fish is calmly using the surface or repeatedly gasping there. One can be normal; the other needs prompt investigation.
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Some species naturally feed, explore, or hold territory near the surface. A fish that remains active, eats normally, and shows healthy colour may simply be using its preferred part of the aquarium. Rapid gulping, especially from several fish at once, changes the picture.
Oxygen enters aquarium water mainly through gas exchange at the surface. A clogged intake, weak filter outlet, or still surface can reduce that exchange. Warm water holds less dissolved oxygen, so a warm, crowded aquarium can become risky faster than a cool, lightly stocked one.
Check ammonia and nitrite because both can affect breathing even when oxygen appears adequate. Verify temperature and inspect every piece of circulation equipment. If injected CO₂ is running, reduce or stop it while investigating; excessive CO₂ can also drive fish to the surface.
When several fish are gasping, increase surface movement promptly with an air stone or correctly positioned filter outlet while testing the water. Supplemental aeration buys time, but it does not remove ammonia, lower an unsafe temperature, or repair failed equipment.
An air pump setup needs tubing, an air stone or sponge filter, and a check valve when the pump sits below the water line. The valve helps prevent back-siphoning during a power outage. Match the pump to the depth and number of devices rather than assuming more airflow is always better.
Once the immediate behavior settles, find the reason it happened. Cleaning a blocked intake, reducing excess heat, correcting water quality, or lowering stocking pressure prevents a repeat; leaving an extra air stone running without addressing the cause only hides the warning.
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