All my fish have died?

WOW ok i think you should add activated aquarium carbon into your filter. Maybe there is some kind of oil that somhow got in to the tank that reduced the amount of air dissolve in the water. that should be the reason why they are gasping for air and the air bubbles cannot burse because of the layer of oil over it. this happen to my tank before and i use a cup to remove the water on top as much as i can. after a few days the oil would disappear on its own.