How to Use Hydrometers in Aquariums

The salinity in a marine aquarium should be close to that of the ocean.

Aquarists use hydrometers to accurately determine the salinity of the water in their marine aquariums. A swing-arm hydrometer -- essentially a hollow acrylic box that contains a needle or pointer that points to a specific gravity and salinity scale printed on the body of the device -- is typically used. The objective is to determine how closely the salt and minerals in aquarium water mimic those found in sea water. Specific gravity is an indirect means of measuring salinity. A specific gravity of 1.025, for example, is identical to a salinity of 35 parts per thousand, the salinity of sea water. Swing-arm hydrometers are typically temperature-corrected and will give an accurate reading in water heated to between 68° and 85° Fahrenheit.

Things You'll Need

• Marine aquarium
• Clear acrylic swing-arm hydrometer
• Dechlorinated fresh water
• Vinegar (optional)

Instructions

1. Condition your new swing-arm hydrometer by soaking it in the salt water from your marine aquarium. Soak the hydrometer for one day prior to use.

2. Dip the hydrometer slowly into the aquarium after the proper amount of conditioning. Ensure that the bottom fill port is below the water's surface. Wait while the water flows upward into the hydrometer and over the weir, the lip which allows the body of the hydrometer to fill with water.

3. Lift the hydrometer up to eye level and tap the acrylic box gently to dislodge any air bubbles that are attached to the pointer. Numerous air bubbles on the pointer will give an inaccurate result.

4. Place the hydrometer on a level work surface and read the salinity and specific gravity scales. You can chose which scale you prefer to work with. The salinity scale will be marked in parts per thousand. Many swing-arm hydrometers have a color blocked "safe zone" printed on the acrylic box and are accurate to a specific gravity of approximately 0.001.

5. Discard the sea water after testing and rinse your hydrometer thoroughly in freshwater. It is important to rinse the hydrometer in order to flush away deposits of salt; these deposits can interfere with your future readings.

Tips & Warnings

• Marine fish and invertebrate animals such as sea anemones require a salinity of 35 ppt.

• Replace water which has evaporated from your marine aquarium with fresh, dechlorinated water. Only the fresh water component of sea water evaporates; the salts remain behind.

• Soak the hydrometer in lukewarm water for up to half an hour to ensure that crystallization does not occur. This should be done every few months. The hydrometer can be soaked in vinegar if the pointer has become caked with mineral deposits, but it must be thoroughly rinsed in fresh, running water thereafter. Leave the hydrometer to air dry after cleaning.

• Rinse your hands to remove soap residue and creams as these are detrimental to the water when working in the aquarium.

References

• Foster and Smith Aquatics: Instant Ocean Hydrometer
• Live Aquaria: How to Maintain Proper Specific Gravity

Resources

• Saltwater Aquarium: Salinity and Specific Gravity