Freshwater fish are NOT adapted to process high levels of solute in the water and would therefore suffer kidney failure if placed into saltwater. The same process happens when you put soft water freshwater fish into hard water tanks, except the process would take longer. read more
Sea water contains a lot of salt. Fish can survive in varying levels of salt, as salt can mess with osmosis of water. If water is salty on one side of a barrier, and pure on the other, then the water will cross the barrier to try to equalise the imbalance. So if a freshwater fish is put in salt water, it will quickly lose water through its body. read more
If a freshwater fish were put in salt water, the extra salt in the water would absorb the water from the fish via its gills until it died. The reverse would happen to a saltwater fish placed in fresh water; it would absorb too much water through its gills and die as a result. read more