Salinity & Specific Gravity

Lion fish in blue aquarium water

The preferred water for marine and reef tanks is RO/DI water, as it is in a very pure form. The commercial salt mix contains all the salts and minerals needed. Salinity describes the content of these dissolved salts and minerals in the water and is measured in parts per thousand (ppt).

The easiest and most common instrument to measure the salt concentration in aquaria is the hydrometer. A hydrometer is a device that measures the density or specific gravity of a liquid. The more salt in the water the more dense it is.

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.

The hydrometer works on the principle that a solid body displaces its own weight of the liquid in which it floats. The hydrometer is calibrated at 60 F (15.55 C) in which distilled water equals 1.000 as the initial point. The readings will rise with increasing density of the water.

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As the instrument is calibrated at a temperature of 60 F (15.55 C), it requires that the water to be tested will also have a temperature of 60 F (15.55 C) in order to get an accurate reading.

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.

Measurements with the hydrometer should be taken below the surface, avoiding air bubbles as not to influence the reading. The hydrometer should be rinsed with freshwater after use to avoid any residues.

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Wayne & Linda Moorefield

JUST STARTING OUT WITH A INHERITED SALT WATER TANK AND WANT IT TO BE PERFECT. WE HAVE ONE NEMO CAME WITH TANK AND WE BOUGHT ONE ANEMONE . WE’RE AT 1.022 ON SCALE AND DON’T KNOW WHETHER TO RAISE IT WITH MORE SALT OR LEAVE IT AS IT IS. ANY HELP WOULD BE GREATLY APPRECIATED . OUR OTHER TANKS ARE FRESH WATER AND THEY ARE FINE, HAD THEM FOR A WHILE.

Thilo@Algone

1.022 is just fine, you can leave that as it is.

andrew r

how does salinity and specific gravity can help a organism

Thilo

Fresh and saltwater fish differ in chemistry, mainly in cell functions. This has to do with osmosis, the fish cells (skin) act as membrane trying to keep the salt levels inside equal to those outside. In essence, water flows out of the fish with higher salinity this is why saltwater fish constantly drink. Freshwater fish urinate more often because water flows into the fish in low salinity environments. The cells are unique to each salt or freshwater fish. They cannot be interchanged. A saltwater fish would literally drown in freshwater and a freshwater fish would dehydrate in saltwater. This is… Read more »

Ethan Cayirylys

are they necessary for a saltwater fish tank

Yes, you most certainly need to have the correct salinity / specific gravity for a saltwater aquarium. It’s not really all that difficult to maintain correct levels, but it’s good to remember changes must be made in small increments.

Soelany

Hi there, We have a salt fish tank the size of 40 gallons. How much should our readings be?
Thanks
Soelany

Generally, salinity of 34 – 36 ppt, or specific gravity from 1.021 – 1.026 are acceptable values.

luvyou521

This helped so much with my school work… thank you….

Erik Sailor

Very usefull table. I’m used for apply correction in my hydrometer set to 15C. Now our ship working only in tropics when average water temp ~29 ~30 C. Thanks a lot. Erik.

Capt. S N Sarangi

Hv been struggling to understand and find this difference when my ship had to declare the salinity of the ballast water onboard and not the sg.
Thank you for the post. For future reference taken screen shot of your table.
Valuable info.
Thanks again

Larry

Thank you. This is a very good reference.

Scott @ Algone

We’re glad you found it useful. Thanks for visiting our site.

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