This statement is not intended to encourage the release of aquarium fish into bodies of water.
Lakes and ponds within cities almost always contain many non-native fish. A pond here in town has 2-3 foot koi--and they most certainly survive the winters. If the water does not freeze solid or winter kill, hardy fish will survive. Wyoming does not have any tropical zones, but there are jack dempseys, mollies and guppies thriving in waters locally. Most are in thermal springs, but the temp still drops below 60 regularly, and these released fish thrive. The fact that they survive is secondary to why is should not be done--aquarium fish can introduce diseases and parasites into the wild, with devestating consequences. If you want to see tropical fish in ponds and streams, travel to their native countries and see them at their best.