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 salt dissolved into the aquarium water by reading in salinity, conductivity, or specific gravity. Most instructions on aquarium salts and other aquarium products refer to specific gravity as the measurement of salinity. The recommended specific gravity for seawater aquariums is 1.024 to 1.026.

Refractometers
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 sends the light higher or lower in the scale's grid. The eyepiece viewfinder is used to determine clearly where your salinity is at 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 just appears in the LCD readout, and additionally it has a temperature-compensating feature (the temperature affects the salinity reading).

Hydrometers
Most novice aquarists will use a hydrometer because it is cheaper, but they are often not as accurate out of the box and are 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 it in the water sample until the container is submersed. 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 they do not lose their calibration, and they are 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 simply have to add or subtract from the reading to get the most accurate result.

EC Meters
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 gives an easy-to-read digital readout, and it 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 what are used to measure salinity in a lab environment. They often have issues in a real-world environment where they are not calibrated with each use, the probes get dirty, and there is electrical interference from equipment wires. For this reason, we always suggest using an EC meter as a secondary measuring device that is 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 you already have one of these controller systems, then we would recommend going with 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 that has the water periodically exchanged; this makes sure that the probe is clean and free of electrical interference.

Calibration Solution
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 to the value you intend to measure as possible. 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 useable in all salinity measuring devices. A solution can have the same conductivity as seawater, making it suitable for an EC meter, but because it was made with synthetic lab salts and not sea salt, it will not weigh as much as seawater or have the same refractive index, making it useless for hydrometers and refractometers. For this reason, our favorite calibration solution is Two Little Fishies AccuraSea. This solution is made using actual seawater, so you can use it to calibrate any type of salinity measuring device. Hydrometers only need to be calibrated once to check the out-of-the-box calibration; refractometers should be calibrated monthly or after they have been bumped or handled in a rough manner. 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.