The preferred water for marine and reef tanks is RO/DI water, as it is in a very pure form. The commercial salt mix contains all the salts and minerals needed. Salinity describes the content of these dissolved salts and minerals in the water and is measured in parts per thousand (ppt).
The easiest and most common instrument to measure the salt concentration in aquaria is the hydrometer. A hydrometer is a device that measures the density or specific gravity of a liquid. The more salt in the water the more dense it is.
It is important to understand that salinity and specific gravity are related - but not the same. Salinity can be measured by (a) boiling down a water sample, (b) measuring conductivity by electronic means, and (c) other laboratory methods which are either too complex or too expensive.
Specific gravity indicates density, while salinity refers to the actual weight of the salt.
The hydrometer works on the principle that a solid body displaces its own weight of the liquid in which it floats. The hydrometer is calibrated at 60 F (15.55 C) in which distilled water equals 1.000 as the initial point. The readings will rise with increasing density of the water.
As the instrument is calibrated at a temperature of 60 F (15.55 C), it requires that the water to be tested will also have a temperature of 60 F (15.55 C) in order to get an accurate reading.
Water will expand or contract if temperatures vary; therefore the density fluctuates with temperature as well. The chart below shows the actual density in relation to the water temperature.
Actual Specific Gravity
A marine tank maintained at a temperature of 82 F (27.77 C) with a hydrometer reading of 1.021 translates into an actual specific gravity of 1.0237
Salinity in ppt
A marine tank maintained at 80 F and a specific gravity reading of 1.022 has a salinity content of 31.8 ppt
Stability in specific gravity is an important factor. Any fluctuation will trigger some effects. Only freshwater will evaporate from the tank leaving the salt and minerals behind. Therefore only freshwater should be used to top off the aquarium.
Further, the water level is also of importance, since the more freshwater evaporates the density the remaining water will increase, thus raising the salinity.
A new batch of salt mix is only required with water changes.
Measurements with the hydrometer should be taken below the surface, avoiding air bubbles as not to influence the reading. The hydrometer should be rinsed with freshwater after use to avoid any residues.