As a marine aquarium keeper, it s your job to maintain an environment as close to what your organisms are accustomed to as possible. Unless your livestock came directly from Utah s Great Salt Lake, too much salinity in your water can be harmful. However, not enough salinity also can make your fish or coral uncomfortable. This is where a piece of equipment called a refractometer comes in handy.

What Is a Refractometer?

Because a refractometer can determine the salinity of your aquarium water, it is a tool worth having on hand, especially if you have a reef aquarium. A refractometer is a piece of equipment that measures the speed of light passing through a liquid, in this case through a sampling of seawater from your aquarium. The more particulates there are in the water, the slower light travels through the water. Depending on where the light falls on the refractometer s built-in scale determines the salinity in your marine aquarium. This also allows you to measure the specific gravity of your seawater. The specific gravity is the density of salt in your aquarium water as compared to fresh water.

The determination of salinity is based on the index of refraction. For example, if you use your refractometer in a vacuum, such as in outer space, the device s index scale should read 1.0000. The index for pure water is 1.3330. For seawater at 35 parts per thousand (ppt), the index reading should be 1.024-1.026. If your refractometer reads higher or lower than 1.024, then you may need to adjust the chemical composition of your aquarium water.

What to Look for When Purchasing a Refractometer

When buying a refractometer, make sure the one you choose is specifically for seawater use. Many refractometers on the market are manufactured only to measure the salinity of sodium chloride solutions. These types are referred to as brine or salt refractometers. They are good for measuring the saltiness of Aunt Sally s blue ribbon pickles, but, as you know, a marine aquarium contains more than just sodium chloride, such as trace elements, calcium, carbonates and magnesium. A seawater refractometer takes these into account.

You also want a refractometer that, once calibrated, automatically takes water temperature into consideration in the reading. Without this automatic temperature compensation (ATC), your reading can be misleading, since warmer water expands slightly, allowing light to pass through more easily. This means your tank of warm water may appear to have less salinity than is actually present.

Other handy aspects of a good refractometer include a dual scale that measures both specific gravity and parts per thousand, and an eyepiece you can adjust in order to clearly read the scale.

How to Read a Refractometer

First, open the cover to the refractometer s prism. Using a clean pipette, take a sample of aquarium water and place a drop or two onto the prism plate, then close the cover. Hold the refractometer to the light and look through the eyepiece, adjusting the eyepiece until you can clearly see the scale. One side of the scale measures the salinity of the water in parts per thousand, which for seawater should read 35 ppt. The other side of the scale lets you know the specific gravity, which is 1.024 for seawater.

As you look through the eyepiece, you will notice that the top part of the scale is blue, while the bottom portion of the scale looks white. Your reading should be taken at the spot where the blue and white meet on the scale.

For the serious marine aquarium person, a refractometer can be a great addition to your tool box, because you can receive more accurate readings than from the traditional hydrometer. But, the refractometer does need to be calibrated in order to give you an accurate reading. Follow manufacturer s instructions to properly calibrate your particular refractometer.