I should've clarified I was mainly referring to how those isolated lakes/ponds were populated with fish originally. Over the timeframes involved since the ice age, even infinitesimally small chances of a rare occurrence can (and do) add up to a "pretty good" chance of that rare occurrence happening.
I did some quick math, and given 100 potential visits per year to an isolated body of water by a bird traveling from another body of water, each with a one in a million chance of transporting eggs, the odds of it having happened at least once in the 11,700 years since the end of the ice age are 69%. In comparison, just to exemplify just how far outside the realm of easy comprehension 11 millennia is, you could play the lottery 100 times a year for your entire adult life (60 years) with the same odds of winning (one in a million) and your odds of winning at least once over those 60 years are only 0.6%.
(Note that these are completely made up numbers for the sake of argument to show that even very unlikely events are bound to happen over long enough time periods.)
I agree that in given the much shorter timeframe involved with the rise and subsequent fall of acid rain, humans are probably much more likely the culprit.
