I think the "Finding Nemo Syndrome" is an urban myth. This week, Australian media reported that toilet-flushed unwanted pets were the source of feral fish. Certainly unwanted pet fish are the source of wild populations. In fact, almost all of about 20 new invasive fish in Australia since the sixties come from the aquarium trade. There is no question that unwanted fish should not be released into streams, dams or any waterway.
Nor should live fish used for bait ever be dumped - carp from Wyangala Dam in NSW used for this purpose were probably the source of a carp infestation in Tasmania which closed two Lakes to anglers for a decade and has cost at least $10 million.
However, are there any fish so hardy that they can survive modern wasterwater treatment to establish a feral population? I strongly doubt it. I couldn't find any reference to an actual case in the literature (not that researchers are likely to have done experiments to test the hypothesis...).
The bottom line is don't put unwanted fish into waterways because they could become feral and don't flush live fish down the toilet because its not an acceptable euthanasia method.