Natural seawater is the most ideal condition for all marine species but is difficult to replicate.  The major element in seawater is salt; the more salt, the higher the density.  Marine water concentration generally fluctuates due to evaporation in a closed tank, thus measuring specific gravity or salinity is important to maintain the right reading for fish to survive.
 

Most aquarist handbooks use this unit of measurement to recommend the amount of salt to be added into a tank. As the hydrometer is calibrated at 15°C, a correlation table must be used to get the correct reading. A simple pointer-hydrometer does not even have a table to make correlations, thus the measurement is highly inaccurate. This is one of the reasons why so many fail in keeping a marine aquarium.  The Marine Salt Testa uses a new method of measuring salinity via the conductivity method which allows users to obtain a reading of salinity in grams per litre. This is usually represented in ppt (parts-per-thousand). The best feature about this method is that readings are automatically compensated despite differing temperatures.  The Marine Salt Testa is an indispensable tool for serious marine aquarists.

Marine Salt Testa Instructions Manual