Salinity

Natural seawater has an average salinity of 34–35 grams per liter (0,26 US.liq.gal.). Most marine organisms have salt tolerances in the range of 30–38 g/l,(0,26 US.liq.gal.) provided that sudden changes are avoided. Theoretically, a saltwater aquarium could therefore be operated in this entire range. However, we only advise very experienced aquarists to move away from the ideal values occasionally (e.g. to increase the oxygen concentration or to reduce the fish’s vulnerability for infections). Any deviation from the ideal values can cause additional physical stress for the tank inhabitants. Always keep in mind that the salt concentration defines not only the sodium chloride (= common salt) but also the concentration of all other substances such as mass elements and trace elements. The optimum salt concentration is at a salinity of 35 ppt.

First of all we should consider the relationship between the salinity and the electrical conductivity. When salts are dissolved in water, electrically charged ions are released and this increases the conductivity of the water. The exact relationship is quite complex, since the ions influence each other and the temperature also plays an important role. Modern conductivity meters automatically calculate the corresponding salinity from the conductivity value, and their measuring accuracy is absolutely sufficient for our purposes. In the laboratory, we measure the salinity with a high-performance laboratory instrument that determines the conductivity of seawater.

If more salt is dissolved in the water, the density of the water increases as well. As a result, it also becomes heavier, and the heavier the salt water is, the lower the relative weight of a body floating in it. With increasing salt concentration, this body will therefore be floating higher, and the salt density can be determined by the swimming height. For this purpose, a hydrometer, a glass spindle with a defined weight and a measuring scale is used. In water, the corresponding density value can be read off the scale at the height of the water surface. For the 35 grams of salt per litre (0,26 US.liq.gal.) we aim for, we read on the scale a value of 1.024 g at 25 °C.

The indication “25 °C” on the hydrometer says that the water measured must have a temperature of 25 °C in order to obtain an accurate value. This is because water molecules expand at higher temperatures and contract when cooled, so that one liter of water has correspondingly fewer or more molecules. All hydrometers are calibrated to certain temperatures, usually 25 °C. The calibration value is noted on the hydrometer.

If, for example, we determine a salt density of 1.024 g/l(0,26 US.liq.gal.), this value is temperature-dependent. If, on the other hand, we measure the corresponding value via the conductivity in microsiemens (53.2 ms/cm) or the salt concentration at 35 ppt, these values are independent of temperature. But beware: Our American friends have here once again their own system, because they measure the specific weight of the water, which at this salt concentration is 1.026 spec. grav. So pay attention when you use American or Chinese products and read their instructions.

Hobby aquarists usually measure the salinity with refractometers. However, we are very unhappy about this, because the incorrect measurements obtained with these devices are a major cause of incorrectly adjusted aquarium systems.

Why is it that way?
The refractometer is an optical device that uses the refractive index of a liquid. You drop the liquid onto the measuring field, look through it and read off a scale to determine the concentration of a certain dissolved substance, e.g. sugar or salt. In itself, this is also simple and reliable – provided that only this one substance is dissolved in the liquid. However, sea salt is not only composed of common salt (sodium chloride), but also contains numerous other salts and is often enriched with vitamins, amino acids or other substances that can influence the refractive index. With the refractometer we do not measure the sodium chloride content, but the total of all dissolved substances, but we interpret this value as a measurement for the sodium chloride.

Meanwhile, there are indeed seawater-calibrated devices available, but also these have to be applied according to the regulations in order to get accurate and reproducible measuring results. The most important thing: Refractometers do not only have to be calibrated, but need to be calibrated with a seawater reference solution that contains all ingredients like the aquarium water. If, on the other hand, these devices are calibrated with distilled water, with osmosis water or a saline solution, one will not get an accurate measuring value and the errors can be up to ten percent. With our product „Multireferenz“, refractometers can be calibrated accurately.

How to use a refractometer correctly?

• Clean and dry the measuring field and the cover flap with distilled water
• Using a clean pipette, add two drops of Multireference to the measuring field and close the cover flap. The transparent cover flap must be completely wetted after closing.
• Stand upright in front of a daylight window or a strong artificial light source, hold the refractometer horizontally in front of your eye and look through it into the light
• Many refractometers allow you to adjust the visual sharpness; focus the instrument so that you can see the measuring scale clearly
• Now turn the calibration screw with a small screwdriver so that the upper edge of the blue surface is at the correct value (see “Salinity reference”)
• Then clean the device again with distilled water (step 1) and apply two drops of the aquarium water to the measuring field
• Always make sure that the complete measuring field is wetted after closing the cover flap. Note down the measured value.

Now repeat steps 1 to 3 to check the calibration again.
If all three measured values are identical, your calibration was correct. You should perform these steps before each measurement.

Measurement with digital refractometer

Here the measurement is similar to that of a photometer. Digital refractometers offer several advantages: They work faster, cleaner and more accurate than manual instruments. The time-consuming sample preparation and cleaning processes of a manual refractometer are eliminated here, because the liquid sample is simply placed on the measuring body for analysis.

One of the biggest factors influencing the refractive index is temperature, since every substance has a specific temperature behavior. Digital refractometers are able to bring the sample to the required temperature via a Peltier element. This not only saves time, but also enables clean, reliable measurements. Digital instruments also offer a much higher accuracy than manual instruments.

Measurement with the hydrometer (commonly also called salinometer)

If you have decided to measure density with a hydrometer, no calibration is necessary. However, we recommend the use of a measuring cylinder, as this allows a more precise measurement than directly in the aquarium where water movements can be particularly disturbing. In addition, there is always the risk of the hydrometer glass body breaking by bumping against the tank glass sheets.

When measuring with a hydrometer it is important to read off correctly. The line on the water surface is used for this, as it marks the measured value on the scale. The surface tension of the water causes the water to rise slightly at the contact surface water/glass, but this is irrelevant for the reading process.

Most hydrometers have a green mark on their scale for the ideal range. As long as the water surface is within this green zone during the measurement and the salt density is kept stable at the same value, it is within the tolerable range.

If you would like to document your measured values to get an overview of the changes, you can obtain tools for creating graphs and tables at the URL www.aquacalculator.com.

Measurement with a conductivity meter (commonly conductivity measurement, conductivity value)

The Practical Salinity Scale (PSS) used today is based on the proportionality of salinity and electrolytic conductivity and is dimensionless. However, the abbreviation PSU, which stands for Practical Salinity Unit, is often used to indicate salinity. This is not a physical unit.

The following formula is used to calculate the salinity from the conductivity:

S=0.0080-0.0941⋅K3215+25.7081⋅K5215+14.3851⋅K15-7.1692⋅K1215+2.0941⋅K3215

Here K15 indicates the ratio of the measured conductivity to the conductivity of a potassium chloride solution of 32.4356 g/kg at 15 °C and one atmosphere pressure. If the ratio K15 is one, S = 35.

In case you own an aquarium computer, you probably know this measuring method quite well. It is carried out with a conductivity/temperature measuring probe directly in the aquarium water and has the advantage that a permanent recording of the measured values is possible. But although the probes are robust today, you should not forget maintenance and regular calibration because deposits and biofilms change the measured values.

With the determination of the conductance, the salinity can now be calculated. This is different depending on the used calculation table and mostly also differs from the measured value of the refractometer, but this difference is negligible. As long as the measuring instruments show a value between 34 and 35 psu or ppt., everything is fine, and you just have to keep this value as stable as possible.

Also the probes of conductivity measuring instruments have to be cleaned and calibrated regularly, but still this measuring variant is very safe and simple, provided that one has purchased a suitable instrument and the appropriate measuring probe. In sea water, the measured values are around 50 mS/cm. Attention: Conductivity meters for osmosis water (TDS) are not suitable for this.

Salt density measurement in the laboratory

Laboratories determine the salt content of liquids in different ways. A calculation method that determines the value by calculating the chlorinity is particularly popular for determining the salinity. This method uses the measured chloride content to draw conclusions about the remaining salts in the water. As a basis, data are available here that have been determined in natural seawater. The basis is that the quantity ratios in natural seawater are the same worldwide.

The Marcet principle of 1819 describes that the ratio of the quantitatively predominant ions (“quantity elements”) of seawater – sodium, chloride and magnesium ions – is identical in all oceans. This principle of constant proportions applies regardless of the total salt content of the respective sea.

However, this is a theoretical calculation, which unfortunately can only be transferred to our aquarium systems to a limited extent. The water in the aquarium does not necessarily correspond to natural seawater due to specific additions that a coral reef aquarium requires. However, the most important ions should be as close as possible to natural values, even in aquaria.

Users of commercially available natural seawater will also not come close to natural conditions, as many products are coastal water (lower salinity and increased mineral concentration due to river water input). In addition, storage and transport in canisters may cause the relations to shift. Please make sure to use only original water from the Atlantic Ocean with full salinity (35 ppt).

Salt density measurements in laboratories are also carried out with the aid of conductivity measuring instruments, which can then be converted to salinity with the aid of corresponding tables. On the page www.aquacalculator.com you will find some tables that make the conversion easier. There are different conversion tables, and depending on the manufacturer and measuring instrument, slightly different results may be obtained when converting from conductivity to salinity. However, these differences are negligible.

Adjust salt density by weight?

Sometimes it is advised not to measure salt density in water at all and simply weigh the salt quantity in order to produce aquarium water with the correct salt density. After all, the manufacturers of sea salt mixtures indicate how many litres of sea water can be produced with the package contents in each case. Just to anticipate: This is sheer nonsense and would be grossly negligent with regard to the safety of the aquarium keepers. But let’s have a look at this in detail.

As mentioned, we aim for a salt content of 34 to 35 grams per liter(0,26 US.liq.gal.) of water. According to the manufacturer of an example salt, approx. 600 liters(158,5 US.liq.gal.) of sea water can be produced from 20 kg (44.09 lbs) of sea salt. This means we dissolve 0.03333333 kg (0,733 lbs) of salt per liter (0,26 US.liq.gal.) of water, or in other words 33.33 grams per liter (0,26 US.liq.gal.). However, we must take into account that a sea salt mixture always has a certain residual moisture. A small part of our sea salt mixture is therefore not salt, but water. After dissolving the salt in 600 liters(158,56 US.liq.gal.) of water, we will not have the desired 34 to 35 ppt when the residual moisture is high, but possibly only 29 or 30 ppt. Put simply, we have 10 % less salt than we thought. Therefore it does not make sense to rely on the weight of the salt.

We would also lack the precise salt density measurement when the aquarium is in operation, because a certain amount of salt is discharged via skimmers, wet filter floss or by reaching into the aquarium, whereas salt can be introduced through food, additives and basic supply systems. A regular and precise measurement of the salt concentration in the aquarium therefore is essential.

For the less erxperienced: No salt disappears from our aquarium with the evaporating water. Simply filling the tank with salt water is therefore not a solution, as this would increase the salt content in the aquarium to levels that are harmful to life.

More about salts, the salt quality and the question why we have developed such special systems can be found in our other HTUs on our download page or our YT channel.