We don't personify fish or otherwise form emotional attachments. That's essentially the entire difference in the horror-less deaths of fish vs horror-filled deaths of dogs.
(It was likely the quick temperature change that killed them.)
(It was likely the quick temperature change that killed them.)
I think it was also the fact that I noticed they were dead before she did. If she just said "oh crap the fish are dead" to me I wouldn't have the same reaction.