Check out this artical that talks about salinity
Salinity is a measure of how much salt is in the water. The standard measure is in parts per thousand (
ppt). Seawater usually has about 28 to 35 parts of salt per 1000 parts of water. As the salt content of water increases, the weight and density of the water also increase. Increased density makes an object float higher. Thus, changes in density can be easily measured as specific gravity with a hydrometer, which floats in the water. The specific gravity of pure water is 1.0000.
Most marine aquarists refer to the salinity of their water in terms of specific gravity, the direct reading on a hydrometer. The true specific gravity of seawater in the 28 to 35
ppt range is 1.0206 to 1.0260, and most marine aquarium systems fall into this range.
It is good to keep the salinity of a marine aquarium a bit lower than ''normal'' seawater. If an old-hand marine aquarist says that he keeps his systems at a salinity of 22, this is short speak for a specific gravity of 1.0220, not 22
ppt — 1.0220 is actually a salinity of 34
ppt at 80 degrees Fahrenheit when measured with a conventionally calibrated hydrometer. A little lower salinity — 1.0210 (at 80 degrees Fahrenheit) or a salinity of 32
ppt — reduces stress on the fish and keeps the tank in the proper range even when salinity increases a bit due to water evaporation. Never add saltwater to a marine aquarium to make up for water lost to evaporation. Only pure water leaves the system; the salt stays behind.