There are many important and essential tools that every saltwater aquarist should have or consider having in their tool box. One such tool such would be one that is important for the success and health of your aquarium is one to measure salinity. There are several different ways that are popular with different hobbyists. Probably the most common and easiest to use would the refractometer.  This device measures the salinity by using the using the refractive index of the water from your aquarium. There are several other devices such as hydrometers and conductivity meters, but in my opinion and experience the refractometer is much simpler. Measuring and keeping an eye on the salinity of your aquarium is important because if the level is too far outside the range of your aquarium inhabitants, it can cause several different problems. Such problems can include a decline in growth, restricted eating, a decline in health, algae blooms, and even the death of your aquarium inhabitants.

How a refractometer works

So let’s talk about how to use you fancy new refractometer. Using it is fairly simple and straight forward; but before we talk about how you use it let’s first talk about it works and how you get an accurate reading from it. The simplest explanation is that the refractometer uses light (sunlight or light from any source) to determine the speed of light passing through your water sample to get the specific gravity which then can be figured into parts per thousand (Holmes 2008). This is accomplished by using the refractive index, also known as the index of refraction (Holmes 2008). The refractive index is the ratio of the speed of light traveling through a vacuum to the speed of light in the material you are testing; in this case you aquarium water (Holmes 2008). Bet you wouldn’t have guessed that you get to use Star Trek lingo/technology to take care of your tank now did ya?

Now that you have an idea of how and why your refractometer works, let’s talk about how you use it to get a salinity reading. First make sure your refractometer is properly calibrated. This can be done by using a standard seawater solution, and then adjusting as needed. Before using a sample from your aquarium (or out in the field), clean the face of the refractometer and the lid so you can get an accurate reading with no contamination from previous samples or dust messing up the readings. Next use a dropper to take a sample from you water and put 2 to 3 drops on the face plate and lower the lid. I would suggest before you use the dropper, take in water from your source a couple a times, that way to get rid of any contaminates from previous samples that may linger in the dropper.

Next make sure you are facing into a light source so you can more easily read what the refractometer is telling you. Now look through the eyepiece, as you can see in Figure 1 you can find your specific gravity and ppt (parts per thousand) by finding where the dark line intersects on the scales on either side of the field of view.

Figure 1 shows how the light gives a reading of specific gravity. Photo curtsey of

Figure 1 shows how the light gives a reading of specific gravity. Photo curtsey of

This is accomplished because when the light travels down into the refractometer, it passes through several lenses and lands on the scale that we see when we look through the viewfinder (Holmes 2008). After you get your reading, I would suggest taking at least one more sample from a different spot in the aquarium just to be on the safe side.

Why it is important to use

As mentioned above and in previous articles, keeping an eye on your salinity levels is very important for the health of your aquarium. Too high of a salinity level can be dangerous and stressful on the organisms in your tank. For example, aquarium snails have a hard time breathing when the salinity reaches 39.8 ppt or 1.030 specific gravity (SG). When salinity reaches 40 ppt it becomes lethal to snails. Other examples include corals turning brown in higher salinity, and an increase in algal blooms above 39 ppt. So by keeping track of your salinity you can keep your tank healthier.


Now, not only do you know how to effectively use your refractometer but you also know how it works and why it works. This is not a product that will break the bank should you decide to get one, prices range from $30 to $60 that I saw by simply doing a google search for it. Not to mention they are small and do not take up a lot of room. I used a refractometer during my studies in marine biology both in the field and in taking care a large saltwater aquarium. It is simple to use and like I said quite small. Since monitoring the salinity levels your saltwater aquarium are so important to the health and continued success of your aquarium, wouldn’t be easier spending $50 dollars to monitor it or spending hundreds replacing the water and the organisms in it?


Holmes-Farley, Randy. “Refractometers and Salinity Measurement”. Reefkeeping. 2008.

“Refractometers Or Hydrometers”. ReefGrowⓇ.  2014.

“Refractometer”. Rollande Bierton. 2014.