As saltwater aquarium keepers, one of the most important water parameters we measure is the salt level: salinity.

While hydrometers are quick and easy to use, they are almost never accurate and tend to worsen over time due to salt and slime build-up. Personally, I would not recommend a hydrometer unless it is calibrated or checked regularly with a refractometer.

A refractometer is a much better way to measure your salinity and a tool that I think every hobbyist should have (or at least have access to). It utilizes a prism to refract the light so it is extremely accurate. There are also no moving parts that can wear out or fail over time so it is as accurate on day 1 as it is on day 1,000.
Here at Marine Depot, we carry two different versions of refractometers: Standard and Pro. Both are equally accurate. The difference is that the more-economical standard refractometer is designed to read a saline/brine solution while the pro refractometer is designed to read natural seawater.
Ideally, the standard refractometer should be calibrated with a calibration solution to compensate for elements other than salt in natural seawater. In actual practice, I have found the difference between calibrating with RO/DI water versus calibration solution to be very minimal.

The pro refractometer is designed specifically to read the salinity of natural seawater so it can be accurately calibrated with regular RO/DI water, distilled water or calibration solution. Additionally, the pro refractometer features a larger scale (ranging from 1.015-1.030) that makes it easier to read.

As a side note, those who have quarantine tanks and use hypo-salinity as treatment will want to use the standard refractometer as a salinity level of 1.009 is required, which is outside the range of the pro refractometer.

With refractomers becoming so reasonably priced and hobbyists understanding the importance of having the correct salinity level, more and more hobbyists are tossing their crusty hydrometers and upgrading to the better and much more accurate refractometers. With properly handling and cleaning after use, a refractometer will provide years of reliable service. Being the holiday season, it also makes a wonderful gift for any saltwater aquarium hobbyist!