Specific Gravity & Salinity Important ?

Instant ocean hydrometer

Water parameters such as PH, alkalinity, ammonia, nitrite, nitrate, etc., are the foundations for a successful and vibrant aquarium and rightfully so, but often neglected and also important is salinity and specific gravity.

What is specific gravity and salinity?

In layman’s terms and without getting too complicated, salinity in an aquarium represents exactly what is sounds like – salt content in the water.  The average salinity in an ocean environment is 35 ppt (parts per thousand). Therefore, for every 1000 grams of water there are 35 grams of salt.  In a reef setup the salinity should be around 34-35ppt.

Specific Gravity represents the amount of dissolved salt in the water. Standard guidelines are varied, but most saltwater aquarists have it at somewhere between 1.020 and 1.025 depending on if its a reef tank or a FOWLR (fish only with live rock).

Some fish only or fish only with live rock tank owners keep it on the lower end. This can help eliminate parasites as most can’t live with SG on the lower end.

Instant ocean hydrometer salinity specific gravity

A calibrated hydrometer or similar is essential for any saltwater aquarist. They are relatively cheap.  Instant Ocean sells one that measures salinity and specific gravity for under $10.


A good hydrometer is especially important when either raising or lowering specific gravity or salinity.  To lower specific gravity use either distilled water or R/O (reverse osmosis).  

Raising specific gravity or salinity involves for lack of a better term – adding saltwater.  Red Sea Coral Pro Salt or similar is sufficient in providing the ideal marine setup.

Red Sea Coral Pro Salt

Whether you have a fish only tank or a reef tank, SG and salinity are equally as important as the other water parameters.









Potters Angelfish bi colored wall hammer coral

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