If the other fish are gasping for air at the water surface, there are several possibilities.

Oxygen deficiency – Please check the oxygen content of the water. If the value is too low (less than 5 mg/l), do a generous water change, check the filter performance. As a short-term measure, it is recommended to install water pumps.

Ammonia/nitrite poisoning – please make sure to also check the ammonium/ammonia level of the tank water, and also the nitrite level. If these parameters are elevated (NH₃/NH₄ > 0.25 mg/l; NO₂ > 0.2 mg/l), this indicates poisoning, which is caused by the bacterial decomposition of the deceased fish. In this case, it is recommended to do a generous water change.