Creating salt water

The most important parameter for successful seawater aquaristics is undoubtedly the nature of the water. Whether the aquarium inhabitants feel comfortable and possibly also multiply, strongly depends on the salt content. One speaks also of salinity and simply of density. In the production of salt water, it is therefore important to adapt the habitat to the requirements of the particular inhabitants and to avoid density fluctuations as far as possible.

As a rule, osmosis water, free of nitrate and phosphate, is mixed with sea salt. With stirring, it is dissolved until it is pH-stable and no undissolved salt residue is present. This process can take several hours. At a water temperature of about 25 °C, a salinity of about 35 is established, which corresponds to most marine organisms. This value is also based on natural seawater which contains about 3.5% salt. With a suitable measuring instrument, the salinity can be checked quickly as a function of the water temperature.

Determination of salinity

Step 1: Measure the temperature

Step 2: Measure the density

Step 3: Determination of salinity using the so-called UNESCO table:1

  Salinitity
  30 31 32 33 34 35 36 37 38 39 40
20 °C 1,02095 1,02171 1,02248 1,02324 1,02400 1,02476 1,02553 1,02629 1,02705 1,02782 1,02858
21 °C 1,02069 1,02145 1,02221 1,02297 1,02373 1,02450 1,02526 1,02602 1,02678 1,02754 1,02831
22 °C 1,02043 1,02118 1,02194 1,02270 1,02346 1,02422 1,02498 1,02574 1,02650 1,02726 1,02802
23 °C 1,02015 1,02090 1,02166 1,02242 1,02318 1,02394 1,02469 1,02545 1,02621 1,02697 1,02773
24 °C 1,01986 1,02062 1,02137 1,02213 1,02289 1,02364 1,02440 1,02516 1,02592 1,02667 1,02743
25 °C 1,01957 1,02032 1,02108 1,02183 1,02259 1,02334 1,02410 1,02486 1,02561 1,02637 1,02713
26 °C 1,01927 1,02002 1,02077 1,02153 1,02228 1,02304 1,02379 1,02455 1,02530 1,02606 1,02681
27 °C 1,01896 1,01971 1,02046 1,02121 1,02197 1,02272 1,02347 1,02423 1,02498 1,02574 1,02649
28 °C 1,01864 1,01939 1,02014 1,02089 1,02165 1,02240 1,02315 1,02390 1,02465 1,02541 1,02616
29 °C 1,01832 1,01907 1,01982 1,02057 1,02132 1,02207 1,02282 1,02357 1,02432 1,02507 1,02583
30 °C 1,01799 1,01873 1,01948 1,02023 1,02098 1,02173 1,02248 1,02323 1,02398 1,02473 1,02548

1 UNESCO Technical Papers in Marine Science 1987

Not only in new aquariums, but also in existing tanks, the ionic composition of salt water must be correct. Marine salts, including chloride, sulfate, sodium, magnesium, calcium, potassium and iodine, are continuously consumed by biological and chemical processes (evaporation, biomass formation, skeletal formation, etc). 

Therefore, the production and maintenance of the natural ion balance with the help of sea salt as well as calcium, magnesium and trace element preparations play a decisive role in the water treatment of seawater aquariums.

Main components of sea water: anions and cations2

  Symbol Average Consentration. g/kg Water bei 35 ‰ Sal. Molarity at 34,8 ‰ Sal. Type & frequency of occurence at 25 °C, 19,375 ‰ Chlorinity, 1 atm.; pH 8
Chloride Cl- 19,353 0,56241 Cl-  
Sodium Na+ 10,76 0,48284 Na+ 97,70%
        NaSO4 2,20%
        NaHCO3 0,03%
Magnesium Mg2+ 1,29 0,0544 Mg2+ 89%
        MgSO4 10%
        MgHCO3+ 0,60%
        MgCO3 0,10%
Sulfate SO42- 2,71 0,02909 SO42- 39%
        NaSO4- 37%
        MgSO4 19%
        CaSO4 4%
Calcium Ca2+ 0,41 0,01059 Ca2+ 88%
        CaSO4 11%
        CaHCO3+ 0,60%
        CaCO3 0,10%
Potassium K+ 0,39 0,01052 K+ 98,80%
        KSO4- 1,20%
Bicarbonate HCO3- 0,14 0,00245 HCO3- 64%
        MgHCO3+ 16%
        NaHCO3 8%
        CaHCO3+ 3%
        CO32- 0,80%
        MgCO3 6%
        NaCO3- 1%
        CaCO3 0,50%
Bromide Br- 0,06 0,00087 Br-  
Boroc acid B(OH)3 0,0046 0,00044 B(OH)3 84%
        B(OH)4- 16%
Strontium Sr2+ 0,0078 0,00009    
Fluorine F- 0,0013   F- 50-80%
        MgF+ 20-50%

2 from Smith 1974, Tab. 1.2-1; 1.2-2; 2-4

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