Actually goldfish can thrive in the wild.
http://fins.actwin.com/nanf/month.9906/msg00090.html

We've joked for years about the pond police coming over the
back fence to haul off our water hyacinths. Who knows, it
may come to pass! (Thinking about the helicopter that was
flying around here the other day and the water hyacinth I
have blooming on the deck....!)

k :-)