Salt water aquariums:Edit
It is best to follow the directions on the package itself, as they will be the absolute most reliable. A key point would be to remember that if you are topping off your aquarium in order to compensate for natural evaporation, use fresh water. Conversely, if water has splashed or spilled out, or if the aquarium has leaked, you would then replace the water with the proper salt water mix. The reason behind this distinction is that when water evaporates from the aquarium - as it will always do - the salt remains behind. If you then add more salt-water mix, you are actually increasing the salt levels in the water.
Fresh water aquariums:Edit
There is a debate as to the sparing use of salt in freshwater aquariums. The idea here being that the bacteria often associated with many fresh water fish diseases can't tolerate salt very well, where as the more complex biology of a fish generally can. It is suggested by some that a ratio of 1 tbsp aquarium salt per 5 gallons of aquarium water can serve as a sort of medicinal soak for the fish. Great care is recommended whenever adding salt to a freshwater aquarium, and only after you have researched the safe levels for your exact fish (ans plants if you have them). There are some species of fish, most commonly the scaleless varieties and almost all freshwater aquatic plants, won't tolerate salt.