Marine Salt has essential trace elements and maintains the PH at 8.2 as well as provide a "replicated" enviroment as the sea. Used for SW aquariums and BW (where the FW meets the SW = BW) Aquarium salt is mainly for treating fish disease, But here is what is stated on the back of API Aquarium Salt:
Benifits: API Aquarium salt provides essentail electrolytes freshwater fish need to reach peak coloration and vitality
When to use: Use API Aquarium Salt when setting up a freshwater aquarium or fish bowl , changing water and treating fish disease.
Directions for use: As a general tonic and stress reducer
Add one rounded tablespoonful for e4very 5 US Gallons or 1/2 rounded teaspoonful for every US Gallon of aquarium water
TO enhance tropical dish disease treatment:
Add one rounded tablespoonful for every 5 US Gallons of aquarium water.
Freshwater fish actively maintain a natural balance of electrolytes in their body fluid. Electrolytes are essential for the uptake of oxygen and the realease of carbon dioxide and ammonia. The lack of electrolytes may cause serious health problems. API aquarium salt, made from evaporated sea water, provides the essential electroytes fish need to thrive in an aquarium.
NOTE: Aquarium salt does not evaporate and is not filtered out, So Aquarium salt should only be added as directed with each water change.