Certain strains of Ebola have a 20% survival rate, so if an outbreak occurs, 4 out of 5 will die. That’s not really evolution happening to the 1 in 5. Whatever saved them was already there...
Perhaps that is why bacteria, as part of their stress response, induce a higher mutation rate. To increase the amount of variation.
As for the fish, over time every variation that made them resistant to the toxin would accumulate in the population.
I doubt that there is just one genetic variation responsible for this, but more likely several.