Why hard breathing points at the water, not just the fish
A fish that is pumping its gills, hanging near the surface or gathering by the filter outflow is telling you the water reaching its gills is not carrying enough oxygen. Most gas exchange in an aquarium happens at the surface, not in the depths, so anything that stills or seals that surface starves the whole tank. Warm water compounds it, because water holds less dissolved oxygen as its temperature climbs, right when the fish's own oxygen demand rises too.
Treat it as a first-response situation: you are buying the fish air while you work out what changed, whether that is a heat spike, a clogged filter, an overfed evening or waste building up faster than the bacteria can process it.
Move one: break the surface
Raise the filter outlet so it ripples or lightly splashes the surface, or lower the water level a fraction so the returning water falls and agitates the top layer. If you keep an airstone, air curtain or spare air pump, get it running now; the rising bubbles themselves matter less than the surface movement they drive. Skim off any oily or dusty film with a sheet of plain paper laid briefly on the surface, because that film is a genuine barrier between water and air.
Check the filter intake for a mat of plant leaves or debris while you are there. A half-blocked intake can halve your flow without the filter sounding any different.
Move two: cool and clean, gently
If the tank is running warmer than its usual setting, bring it down slowly, a small amount at a time, by easing the heater setting or floating a sealed bottle of cool water, never by dumping in cold water, which shocks already stressed fish. Stop feeding for now; uneaten food and the extra waste it becomes both consume oxygen as they break down.
Test the water once the fish look a little easier. Waste compounds can irritate gills and force hard breathing even where oxygen itself is adequate, and a partial change with conditioned, temperature-matched water is the right follow-up if your readings are off. Make that change a modest one and pour gently, since fish working hard to breathe do not need a current to fight.
When first response is not enough
If the whole tank is gasping despite a rippling surface and running airstone, or a fish is lying on its side, losing balance or breathing hard on its own while tankmates look normal, you are past home measures. A single fish struggling in good water may have a gill or internal problem, and several fish still struggling in improved water suggests something in the water you cannot see. Either way, an aquatic vet or a well-equipped fish shop that can test a water sample promptly is the next call, and sooner rather than later.
Write down what you observed, what you changed and how the fish responded in your fish's Pet Passport on KinsLe, because a vet reading that timeline can tell far more from it than from a worried summary at the counter.