Maintaining a saltwater aquarium means trying to mimic, as close as possible, the natural composition of seawater. The most basic parameter to do this is salinity. Salinity is defined as the amount of dissolved salts in water. Natural seawater has a salinity of roughly 35 ppt (parts per thousand by weight), so this is the number you are shooting for. This means that there are 35 grams of salt in a liter of seawater. Measuring the salinity of your aquarium directly is not very convenient as you would have to take a direct weight measurement of all of the water in the tank, and then evaporate off all the water and weight the residue. As an alternative, we are using a salinity refractometer. What is a salinity refractometer? This device measures specific gravity which is the ratio of the density of the saltwater sample to the density of pure water at the same temperature. There is a lot of science involved with this that makes my brain hurt studying it so I will keep it simple.
To use a refractometer, you place 2-3 drops under the daylight plate. Light enters, refracts off the saltwater, passes through lenses, and lands on a scale.
The values on the right are “0/00” which stand for ppt. This makes it easy to identify the 35 ppt we are shooting for. The values on the left are the specific gravity measurement, in which the target value is 1.026. The tolerable salinity range for invertebrates is roughly 34-35 ppt, yet fish can tolerate lower salinity. These values are target values, but the true salinity of your tank may fluctuate due to evaporation. When water evaporates from the tank, it is only the water that leaves the tank, making the remaining water more concentrated. To correct for this, pure water needs to be added to compensate. We add approximately 1 gallon of RO/DI water per day due to evaporation. You do NOT want to add mixed saltwater, as you would be making the salinity increase.
We check the tank’s salinity regularly, and for each batch of new saltwater made. We wait for the salt to dissolve so we can get the most accurate reading before placing the fresh saltwater into the aquarium.