How long can a salt water fish live in just plain water?

A salt water fish cannot survive in just plain water. This is because salt water fish have evolved to live in an environment with a high salt content, and their bodies are adapted to this environment. When a salt water fish is placed in just plain water, its body will begin to lose salt and water, and it will eventually die. The length of time that a salt water fish can live in just plain water will vary depending on the species of fish and the temperature of the water, but most fish will only survive for a few hours.