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How to Maintain Proper Specific Gravity

How to Maintain Proper Specific Gravityr

Specific gravity measures the relative salinity (amount of dissolved salts) in your aquarium water in comparison to pure water. Measuring specific gravity on a regular basis is crucial to maintain the ideal environment for your marine inhabitants. The following helpful tips will ensure accurate results when measuring specific gravity.

  • Most hobbyists use a simple hydrometer or a refractometer to measure salinity.
  • For professional results consider a portable refractometer or a high tech electronic salinity monitor.
  • Before changing your aquarium water, measure and record the specific gravity. Make sure the new saltwater matches the specific gravity of your aquarium. Too much of a deviation in salt levels could "shock" or potentially kill sensitive aquarium inhabitants.
  • Replenish evaporated water with clean freshwater - not prepared salt water. Remember, salts remain behind after water evaporates. If you top off your aquarium with salt water, you are compounding salt levels in your aquarium.
  • Before using a new swing-arm hydrometer, condition or "season" the hydrometer by soaking it in saltwater for 24 hours.

Recommended Specific Gravity Levels
Reef Aquarium: 1.023 to 1.025
FOWLR Aquarium: 1.020 to 1.02


What is a refractometer?

Serious hobbyists know that the best way to measure the salinity and specific gravity of aquarium water is to use a refractometer. These precision optical instruments are incredibly accurate even at very low salt levels, and are equally easy to use. Recently, these instruments have become more affordable, so now every aquarium owner can have one to help monitor water conditions.

How do I use it?

Place 1 to 2 drops of aquarium water onto the prism glass and close the plate cover. Once a fluid sample is placed on the measuring surface, the light passing through it slows and bends. The refractometer focuses this bent light on a tiny internal scale. The scale is magnified by the eyepiece lenses so it is easily visible.

Look through the eyepiece and view the scale. The top portion of the scale appears blue and the bottom part appears white. Take your reading at the intersection of these two colors. One side of the scale measures salinity and the other side measures specific gravity.

To effectively monitor your marine aquarium water conditions, consider investing in a refractometer. The accuracy and affordability make it worthwhile.

Salinity & Specific Gravity are easily viewed when looking through the Refractometer


Accurately test your aquarium's salinity with a Portable Refractometer. Simple to use, optical instrument gives a direct, temperature compensated reading of specific gravity and salinity.

Swing-arm styled Instant Ocean Hydrometer economically tests your aquarium's salinity and specific gravity.


Salinity Monitors
Salinity Monitors
Salt, Conditioners & Additives
Salt, Conditioners & Additives



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