Every saltwater aquarium requires a testing instrument to measure the salinity (salt level) of the water. To do so, you will need either a hydrometer, such as the Coralife Deep Six Hydrometer; a refractometer, such as the MarineAndReef.com REEFractometer; or an electrical conductivity (EC) meter, such as the Hanna Salinity Tester. These devices measure the amount of dissolved salt in the aquarium water by measuring salinity, conductivity, or specific gravity. Most instructions for aquarium salts and other aquarium products use specific gravity as the measure of salinity. The recommended specific gravity for seawater aquariums is 1.024 to 1.026.

For many years hydrometers were far more common than refractometers, but refractometers are becoming more common. We recommend using a refractometer because it is more accurate than a hydrometer and less prone to technical errors than an EC meter. To use, just put a drop of your saltwater onto the refractometer’s glass plate. The refractometer uses ambient light to measure the saltwater’s refractive index. As the light shines through the refractometer, it passes through lenses and appears on a scale. The light bending at the prism shifts it higher or lower on the scale's grid. The eyepiece viewfinder is used to determine clearly where your salinity is on the refractometer’s scale.

The Milwaukee Instruments Seawater (Salinity) Refractometer is easier to use than a traditional refractometer because you do not need to read the scale through the eyepiece. The reading appears on the LCD readout, and it also has a temperature-compensating feature (temperature affects the salinity reading).

Most novice aquarists will use a hydrometer because it is cheaper, but it is often not as accurate out of the box and is prone to user error when measuring. To use a hydrometer, you will first want to break it in by filling the unit with saltwater and letting it sit for 24 hours. Then, to measure your saltwater, you should dip the container into the water sample until it is fully submerged. When it is full, set it on a flat surface. Sometimes there will be air bubbles on the meter—make sure to tap those off by lightly tapping the hydrometer. Leaving air bubbles in the hydrometer is the most common cause of user error. When the hydrometer is full, the buoyant needle will float to your saltwater level. If it reads high, dilute the mixed water with more fresh water, and if it reads low, gradually add salt mix. When you are done with the hydrometer, rinse it in fresh water and store it in a dry place. The Instant Ocean Hydrometer is made of plastic and is affordable and accurate enough for basic aquarium testing. An advantage of a hydrometer is that it does not lose its calibration and is very consistent when used properly. However, hydrometers are often off by 1-2 points out of the box, and there is no way of mechanically calibrating them. You will have to add or subtract from the reading to get the most accurate result.

Some aquarists prefer an EC meter, or electrical conductivity meter, to check their salinity. When the EC probe is placed in saltwater, the meter applies voltage between two electrodes inside the probe. The saltwater's electrical resistance causes the voltage to drop, which the meter then reads. The advantages of an EC meter are that it provides an easy-to-read digital readout and can be very accurate. EC meters can also be left in an aquarium for constant salinity monitoring. Our experience is that, while these meters are accurate, they are used to measure salinity in a lab environment. They often have issues in real-world environments where they are not calibrated with each use, the probes get dirty, and there is electrical interference from equipment wiring. For this reason, we always recommend using an EC meter as a secondary measuring device, occasionally checked against a hydrometer or refractometer. For a handheld option, the Hanna Salinity Tester is our preferred option. For constant monitoring, both Hydros and Neptune have salinity probes. If you want salinity monitoring and already have one of these controller systems, we recommend using that system's salinity option. Please note that you will not just need a probe but a module that has a salinity probe port. For Neptune Systems this is the PM2 module, and for Hydros this is the Hydros Launch or Hydros X10. The most accurate salinity monitoring we have tried is with the Hydros Salinity Kit and the Hydros iV. The iV, or Isolation Vessel, keeps the probe in a small beaker whose water is periodically exchanged; this ensures the probe is clean and free of electrical interference.

With whatever measuring device you choose, it is a good idea to calibrate the device or check it against a standard to make sure it is working. While you can calibrate refractometers with purified water, it is always best to calibrate as close as possible to the value you intend to measure. Multiple manufacturers make calibration solutions for their devices, including Neptune, Hydros, and Hanna. It is important to keep in mind that not all calibration solutions are usable in all salinity measuring devices. A solution can have the same conductivity as seawater, making it suitable for an EC meter. Still, because it was made with synthetic lab salts rather than sea salt, it will not weigh as much as seawater or have the same refractive index, rendering it useless for hydrometers and refractometers. For this reason, our favorite calibration solution is Two Little Fishies AccuraSea. This solution is made with actual seawater, so you can use it to calibrate any salinity-measuring device. Hydrometers only need to be calibrated once to verify the out-of-the-box calibration; refractometers should be calibrated monthly or after they have been bumped or handled roughly. EC meters should be calibrated monthly for pen-style meters. Leave-in monitors should be calibrated at least twice a year with periodic checks against other measurement devices to confirm they are reading correctly. It is common for leave-in probes to be 1-2 points off even after calibration in real-world situations.