because if a saltwater fish is in fresh water it cannot be alive because of the food from that type of water if the don't live in the right kind of water then why do you think they were born in that...
Very few fish can make the transition for any length of time. A Molly can successfully do it. There are brackish water fish that eventually go all salt.
Not long. In some species a few minutes in others maybe an hour or so and in others maybe a few days. It would be a terribly painfull way for the fish to die and extremely cruel of anyone to do such...