Different rivers, oceans, and seas have different amounts of salt and, yes, even freshwater contains some salt. How much salt is dissolved in the water determines the salinity of a particular water sample.

Essentially, there are three types of water: low salinity or freshwater; medium salinity or brackish tidal waters; and high salinity, which is seawater. And even in seawater, the amount of salt can vary dramatically.

Water is normally measured as parts per thousand of a liter: seawater normally has a reading of 33 - 37, but salinity is not easy to measure and has a close relationship with specific gravity (see below). Fish have evolved to live within specific levels of salinity, and a rise or fall in the water's salinity can change a fish's osmoregulatory process, thereby causing stress.

When preparing your saltwater tank, you need to calculate the volume of water needed to fill your aquarium, keeping in mind that decor takes up space. It is essential that this measurement is accurate. Pour water directly into the tank (in order to protect your décor and aqua-scaping, place a small saucer or bowl into the tank and pour the water directly into that). After the tank is filled to the desired level, add the appropriate amount of water conditioner and/or additive. Then, start adding your commercial marine salt, a little at a time, until you reach the correct salinity level; it could take up to 24 hours for the salinity level to stabilize so wait before making any adjustments.

When doing a water change the salt should be completely dissolved before being added, so that the proper salinity is reached immediately. When topping up, only use pure water; no salt is lost due to evaporation so do not add any.

Check the salinity level of your tank after initial filling and after every water change and evaporation top up.

Specific Gravity

Measuring specific gravity with a hydrometer is the best way of checking the salinity of your water; seawater has a specific gravity reading of between 1.021 and 1.035 depending on the temperature and location. Specific gravity alters with temperature, however, so be sure that you purchase a hydrometer that can make adjustments to compensate for temperature variation. If the reading is too low, add more pre-mixed "salt" water; if it is too high, add more fresh water to the tank.