I'm fairly new, my thought is either heavy metal toxin or contaminated fish food. As for stress or water I don't think so, from my pass experience I would say that I stressed my fish to the MAX. MY tank pump float jammed...to make a long store short. I lost about 840 gals of good fish water when my 850 gal tank lost all but 3/4" of water (check out "Metal Core on pump" by Apollo). It did slow their eating and activity down but only for 2 days, NO FISH DIED!

I know that the person I bought my tilapia from puts his fish food into freezer baggies then straight into the freezer as soon as he opens the large bag. As a safety precaution, says that it can spoil over time.