Basically, the reason why freshwater aquarium salt is needed is because it helps to reduce the slime coat on the scales and skin to allow better penetration of fish medication. Other than treatment of disease, adding aquarium salt is also beneficial in terms of prevention of disease because slight salinity will help to control growth of protozoa. Usually for treatment purposes, salt concentration of 0.5% should be sufficient and this can be achieved by adding 1 tablespoonful to every gallon of water that you have. However, if you intend to add only just for prevention, 1 tablespoonful for every 5 gallon is more than enough.
Before you add in any aquarium salt, first of all, remove all the live aquarium plants. A correct method of adding to your aquarium is to dissolve it in a bucket of freshwater and then slowly pour in the water. Never throw the salt directly into the aquarium without first dissolving because some curious fish can actually tried to gobble up and taste it like fish food. Beware, certain tropical fish species like the catfish do not tolerate salt in the water and therefore you should check carefully before you do anything.
Marine Salt on the other hand, is added into your saltwater aquarium to create the same level of water salinity just like natural seawater for the survival of marine fish. It also acts as a carbonate buffer that maintains high pH at a certain level and a good marine salt should contain all the major ions in the right proportion just like natural seawater.
Usually a good rule of thumb is to maintain the salinity level at between 1.020 to 1.023 for fish-only marine tank while for those reef tanks, a slightly higher specific gravity of 1.023 to 1.025 is more suitable. There is also another type of aquarium salt known as reef salt which contains trace elements like calcium for proper growth of aquarium reefs.
Other related aquarium fish supplies: Water Test Kit and pH Test Kit