Understand Salinity and Specific Gravity of Saltwater in Reef Aquarium

Reef salt salinity test
Seasalts

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.

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

Specific Gravity 79 F (26.11 C) 80 F (26.66 C) 81 F (27.22 C) 82 F (27.77 C) 83 F (28.33 C) 84 F (28.88 C) 85 F (29.44 C)
1.017 1.0193 1.0194 1.0196 1.0197 1.0199 1.0200 1.0202
1.018 1.0203 1.0204 1.0206 1.0207 1.0209 1.0210 1.0212
1.019 1.0213 1.0214 1.0216 1.0217 1.0219 1.0220 1.0222
1.020 1.0223 1.0224 1.0226 1.0227 1.0229 1.0230 1.0232
1.021 1.0233 1.0234 1.0236 1.0237 1.0239 1.0240 1.0242
1.022 1.0243 1.0244 1.0246 1.0247 1.0249 1.0250 1.0252
1.023 1.0253 1.0254 1.0256 1.0257 1.0259 1.0260 1.0262
1.024 1.0263 1.0264 1.0266 1.0267 1.0269 1.0270 1.0272
1.025 1.0273 1.0274 1.0276 1.0277 1.0279 1.0280 1.02

Example:
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

F (C) 1.020 1.021 1.022 1.023 1.024 1.025 1.026 1.027
74.0 (23.3) 28.0 29.3 30.6 31.9 33.3 34.6 35.9 37.2
75.0 (23.9) 28.2 29.5 30.8 32.1 33.5 34.8 36.1 37.4
76.0 (24.4) 28.4 29.7 31.0 32.3 33.7 35.0 36.3 37.6
77.0 (25.0) 28.6 29.9 31.2 32.5 33.9 35.2 36.5 37.8
78.0 (25.6) 28.8 30.1 31.4 32.7 34.1 35.4 36.7 38.0
79.0 (26.1) 29.0 30.3 31.6 32.9 34.3 35.6 36.9 38.2
80.0 (26.7) 29.2 30.5 31.8 33.2 34.5 35.8 37.1 38.5
81.0 (27.2) 29.4 30.7 32.0 33.4 34.7 36.0 37.4 38.7
82.0 (27.8) 29.6 30.9 32.3 33.6 34.9 36.3 37.6 38.9
83.0 (28.3) 29.8 31.2 32.5 33.8 35.2 36.5 37.8 39.2
84.0 (28.9) 30.1 31.4 32.7 34.1 35.4 36.7 38.1 39.4
85.0 (29.4) 30.3 31.6 33.0 34.3 35.6 36.9 38.3 39.6
86.0 (30.0) 30.5 31.8 33.2 34.5 35.8 37.2 38.5 39.8
87.0 (30.6) 30.8 32.1 33.4 34.8 36.1 37.4 38.8 40.1

Example:

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.

 

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