Salinity is not usually of great importance in a tropical freshwater aquarium. The salinity can be measured by the total amount of dissolved salts in a given sample and is usually described in parts per thousand (ppt). Fresh water has a salt content of no more than 1ppt and seawater has a salinity of 35ppt.
Specific gravity is a measure of the weight of water, which will fluctuate according to the amount of salts it contains. The more salt dissolved within, the denser water becomes and it's specific gravity rises. Pure water has a S.G. of 1 while seawater has a S.G. of between 1.021 and 1.024.