Instructions
Purchase a hobbyist grade hydrometer from your local pet shop. A hobbyist grade hydrometer is calibrated to work at the temperature of your fish tank. This is the only way to get truly accurate results. The easiest hydrometers to use look like a small clear acrylic picture, with a swing-arm located in the center. Once full of water this swing-arm will point to a number marked on the side of the hydrometer. This number is your specific gravity. This type of hydrometer will have a specific gravity scale between 1.0 and 1.030.
Collect water in the hydrometer from your aquarium. At this point your aquarium should be at the appropriate operating water level and temperature. Simply hold the handle of the hydrometer, and slowly dip the mouth into your tank. You want to collect water from the center of the tank, not the surface or the edges. Once submerged the hydrometer will fill with water. When full, remove the hydrometer from the water, and place it on a flat surface.
Remove any air bubbles from the hydrometer. You will not get an accurate reading if you see an accumulation of air bubbles on the swing-arm insides of the hydrometer. By lightly tapping on the sides of the hydrometer these air bubbles will release to the surface.
Get a reading from your hydrometer. At this point the swing-arm should be pointing to a number, within the range it measures, indicating your aquarium's specific gravity. The optimal specific gravity of a saltwater aquarium should be within the range of 1.02 and 1.026. If your aquarium is under 1.020, then you will need to add salt solution. If you are over 1.026, then you will need to add freshwater to your fish tank.
Clean your hydrometer. Rinse your hydrometer out under running tap water for 15 to 20 seconds. It is very important that you clean your hydrometer in between uses. This will assure that you get accurate results.
How to Use a Hydrometer in a Saltwater Fish Tank
A hydrometer is not only an extremely important piece of equipment for a saltwater aquarium owner, but it is often a much overlooked tool for many novice owners. Hydrometers simply measure the density or specific gravity of a liquid. In a saltwater aquarium this density is dictated by its salt concentration. The more salt the water contains, the denser it is. Knowing the specific gravity of your aquarium is extremely important for maintaining the optimal health of your fish. While most marine life will acclimate themselves to almost any specific gravity within a certain range, variations within this range can trigger some harmful effects.