Fish gasping at the surface
Fish at the surface, mouths breaking the water, are breathing wrong. The whole tank doing it at once is almost always water: low oxygen, high ammonia, or high nitrite. One fish doing it while the others feed can be gill flukes, injury, or the start of the same water problem. Test before you medicate.
Do this in the first hour
Look at the thermometer and the filter. A heater stuck on, a filter that stopped, or a film of dust on the surface can drop oxygen fast. Increase surface movement with an air stone or by lowering the water to the filter outlet so it splashes. Do not crank the heater to "help." Match any new water to the tank temperature.
Change 30 to 50 percent of the water if you do not have test results yet and many fish are gasping. Use a dechlorinator that treats the tap water you actually have. Siphon debris from the gravel without tearing up the whole bed. Ammonia spikes after overfeeding, a dead fish, a crashed cycle, or a filter rinsed in chlorinated tap water until the bacteria were gone.
Read the tests
Ammonia and nitrite should be zero in a cycled tank. Any measurable ammonia with gasping fish is an emergency for the animals even if they are still upright. Nitrite damages gills and blood. Nitrate is a slower stressor. pH crashes happen in poorly buffered tanks and will also put fish at the surface. Write the numbers down. "The water looks clear" is not a test.
If ammonia is high, keep changing water, stop feeding for a day, and do not add an antibiotic as a reflex. Medicines that claim to fix water are not a substitute for removal of the waste. Some "pH up" products swing the chemistry harder than the fish can stand.
When it is not the whole tank
A single fish clamping fins, flashing, or gasping while tankmates are normal may have parasites or a bacterial gill infection. Quarantine is ideal if the fish can be netted without a chase that uses up the last of its oxygen. A short net, a wait, and a container of tank water beat a ten-minute pursuit. Salt, formalin, and copper are easy to overdose. Copper is deadly to invertebrates and to some fish, and it does not belong in the display tank "just in case."
Marine fish gasping is the same emergency with less room for error. Check salinity, temperature, and whether a protein skimmer or powerhead failed. The checker marks widespread gasping as urgent. Freshwater and marine profiles are on the fish directory and the marine directory.
Sources
Clinic decisions belong to the veterinarian who can examine the animal. This page is background reading.
Gasping fish questions
Why are fish gasping at the top?
They are usually short of oxygen, or gills are damaged by ammonia, nitrite, or parasites. Test the water before adding medications.
Should I do a water change?
A partial water change with temperature-matched, dechlorinated water is the usual first step when many fish gasp at once.
Is one fish gasping different?
One fish at the surface can still be a water problem. It can also be gill disease in that fish. If the group looks fine, separate the sick fish only if you can do it gently, and test the tank anyway.