How to Use an Aquarium Hydrometer

Maintaining proper specific gravity is paramount to the success of a saltwater aquarium. Marine water is very complex and the amounts of salt dissolved can vary greatly depending on the ecosystem. A hydrometer suitable for aquarium use can take the guesswork out of keeping the aquarium's chemistry in order. Hydrometers do not actually measure the salinity of the tank, but the specific gravity--essentially everything that is dissolved in the water, according to the Non Destructive Training Resource Center. Hydrometers also come in three different types: floating, swinging arm and submersible.

Things You'll Need

  • Hydrometer
  • Marine aquarium
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Instructions

  1. Floating Hydrometer

    • 1

      Thoroughly wash the outside of the hydrometer before placing it in the tank to prevent foreign chemicals from leeching off into the water.

    • 2

      Carefully drop the hydrometer into the water column, then wait a few seconds for the hydrometer to stabilize.

    • 3

      Read the measurement lines, located on the side of the hydrometer, at the point where the surface of the water is touching.

    Swinging Arm Hydrometer

    • 4

      Rinse the inside of the hydrometer with warm water, taking care not to dislodge the measuring arm. Salt build-up from evaporation can skew results in this type of hydrometer.

    • 5

      Gently dip the hydrometer into the water, filling the hydrometer. Take care not to allow bubbles to stick to the floating arm.

    • 6

      Set the hydrometer on a hard, flat surface and allow the arm to adjust. When the arm has stopped moving, take note of where the arm is pointing.

    • 7

      Look to the measurement lines to see the specific gravity and salinity. On an Instant Ocean hydrometer, the specific gravity measurements are located on the right, while the salinity measurements are on the left. On the Red Sea, the specific gravity measurements are located on the outside edge of the unit.

    Submersible Hydrometer

    • 8

      Rinse off the hydrometer to remove any impurities that may have carried over from the shipping or packaging of the hydrometer. Also make sure to clean the inside so that the results will be accurate.

    • 9

      Submerge the hydrometer into the water at an aesthetically pleasing spot without too much water movement. Stick the suction cup to the glass.

    • 10

      Read the hydrometer by taking measurements from where the arm is pointing. On the submersible Red Sea hydrometer, the results are on the outside edge, while the inside is temperature.