Saltwater fish are able to survive in freshwater, but they can't live in it permanently. When a saltwater fish is placed in fresh water, its body will start to absorb the freshwater. This causes the salt concentration in the fish's blood and tissues to decrease. The freshwater will also dilute the fish's digestive juices, making it harder for the fish to digest food. If a saltwater fish stays in fresh water for too long, its body will eventually absorb so much freshwater that it will die.