Actually, if we go with Nate's suggestion about poor oxygen exchange (which I would agree with btw) this could explain the heavy breathing and death. It could be that your fish were just barely getting by with the poor oxygen level in the tank. Then one day you add a powerhead which introduces more flow which forces the fish to swim more which causes them to breath heavier consuming more precious oxygen which is already lacking in the tank and releasing deadly CO2 and suddenly you have a series of unfortunate events
