Why should you once in a while add salt to a freshwater aquarium. Even though most times it should not be done unless you have brackish fish like mollies. Salt can help reduce stress on the fish, add hardness to the water if it gets to soft. As well as help medication travel the fish tank easier. Unless the fish are brackish you should add salt sparingly. I do not recommend to use salt in your freshwater aquarium since this could have very adverse effects on the aquarium ecosystem and have a huge chance of killing your fish. Unless they are brackish like Mollies or even Archer fish.