If all of your fish are gasping at the surface then that means that there is not enough surface agitation... So.. where is your filter OUTPUT? If it is below the water level then you must move it up to where it breaks the surface. It doesn't have to noisy or violent, it just must break the surface tension of the water. In effect the still water is like "stagnant" and the fish are not getting oxygen. Adjust the output and by morning your fish will be back swimming below and breathing properly. Good luck.
Let us know what you find.
I know you said the water was perfect, but what were the specific test results for ammonia, nitrite and nitrate? Were you using a liquid test kit or strips?
Which tank was this that had the crash? What were the inhabitants?
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