There are a lot of tropical fish that will actually survive in the cold water over the winters we have. When I lived in Tennessee, a feller caught a 32 pound Pacu in Old Hickory Lake in Nashville, TN. Everyone thought for sure the tropicals would die in that lake since it gets down in the upper thirties every winter. That one had survived for many years.
When you catch an obvious tropical you need to remove it and dispose of it.