Was the canister filter not on or working and then was turned on after a long time without power? If so and it was dirty inside and hadn't been cleaned in some time that would release poisonous gasses into the tank.
Any time you see fish gasping at the surface I highly recommend doing an instant water change and make it a large water change. More then 50 to 70%. It normally means there is a lack of oxygen or there is something contaminating the tank and the fish are basically suffocating.
How long has the tank been running?
Did you test for nitrite? Do you run co2?
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