So, you have decided to set up a saltwater tank?
Saltwater tanks are a lot of work, and mixing salt water will be a major part of your life now.
While it may seem simple, mixing salt water is an art.
If the salinity levels are too low or too high, your fish will become stressed and, in extreme cases, die.
That is why measuring salinity is a vital part of owning a saltwater tank.
The best tool for this is a refractometer.
If this is your first time hearing about refractometers, don’t panic!
Today, I am going to teach you everything you want to know about this salinity measuring device.
What is a refractometer, and how does it work?
A refractometer is a tool that accurately measures the salinity of your aquarium.
Those of you who paid attention in school might recall your science teacher talking about refraction of light.
If you didn’t pay attention in class like me or just can’t remember – here is a simple refresher.
Light travels in a straight line.
But when it shines through a medium like glass or water, it changes direction or “bends.”
Just how much the light bends is determined by the medium that the light shines through.
Take this glass of water for instance…
Notice how the lines of the backdrop appear differently when viewed through the glass?
This bending of the light is referred to as refraction.
And, it turns out that you can use refraction to determine the salinity of your aquarium.
You see, light refracts differently according to the amount of salt that is dissolved in the water.
A refractometer can “read” these differences, allowing you to accurately measure the salinity of your tank.
Which refractometers are suitable for aquariums?
I’m going to let you in on a little secret that many expert reef keepers don’t even know…
Many hobbyists use the wrong refractometer when testing the water in their tank.
You see, the majority of salinity refractometers are designed to measure the amount of sodium chloride (NaCl), more commonly known as salt.
But seawater is more than just sodium chloride – it also contains other elements like magnesium and calcium.
And most salinity refractometers are not calibrated for these extra elements.
This means that a standard refractometer can produce an error of up to 5% when measuring salinity.
But if you have already own a salinity meter, don’t worry, I have some good news…
While this margin of error is significant, it shouldn’t be so big that your aquarium fails because of it. In fact, not only have I seen many coral sellers use an incorrect refractometer and produce amazing specimens, I’ve seen thriving reef tanks based on those readings too.
But for those of you who want the most accurate salinity measurement possible, you need to buy a seawater refractometer instead of a saltwater refractometer.
What are the different types of refractometers?
There is a wide range of refractometers on the market capable of testing a variety of different substances like urine, blood, wine, honey, antifreeze, beer…
And while they all look the same, they are not interchangeable.
So, make sure you double check that the refractometer you are buying is used to measure salinity.
I have seen many newbies frustrated because they bought the wrong refractometer – don’t make the same mistake.
When it comes to fishkeeping, there are two types of refractometers that are commonly used:
1. Handheld salinity refractometers (analog)
This is the cheapest and most commonly used aquarium refractometer.
Simply calibrate the refractometer, add a couple drops of aquarium water to the refractive prism and stare into the eyepiece like a pirate to get the salinity accurate reading.
While cheaper models may not be as precise as the digital type, an analog refractometer will be accurate enough for the majority of aquarists.
I recommend choosing a handheld refractometer with automatic temperature compensation (ATC), which allows for a more accurate measurement.
Analog seawater refractometer displays salinity measurements in parts per thousand (PPT) and specific gravity (SG).
- Doesn’t need batteries
- Cheaper models are less accurate
- Requires more effort to use
- Manual calibration
- Subjective measurements
- Needs a bright light source
2. Digital salinity refractometers
Comparing a digital refractometer to a handheld one is like comparing a horse and cart to a car – a digital refractometer does the same thing but makes the entire process much easier.
Digital refractometers are simple to use and return a precise salinity measurement in just a few seconds.
And because the measurement is clearly displayed on the LCD screen, you do not have to interpret the results yourself.
Of course, all these wonderful features come at a price, and digital refractometers are more expensive then the handheld variety.
- Easy to use
- Require batteries
- Bright light can cause errors
Which are the best refractometers?
Best handheld refractometer for the money – Ade Advanced Optics
This is a cheap refractometer that is popular with beginner aquarists setting up a new tank.
With built-in automatic temperature compensation (ATC) and included accessories like cleaning cloth, pipette and storage case, this refractometer offers incredible value for the money.
If you are new to the hobby and just want a reliable refractometer at a budget price, look no further.
Best handheld refractometer for accuracy – Vee Gee
|Measurement scale||Salinity PPT (0-100%), SG 1.000 – 1.070|
|Accuracy||Salinity PPT +/- 1.0%, SG +/- 0.001|
Vee Gee’s analog refractometer operates exactly the same as the cheaper models but is much more accurate. And the build quality is great, rugged and durable. Cheaper refractometers feel like toys in comparison.
The problem is that these units are expensive. And unless you like to do things the old-fashioned way, I suggest buying a good digital refractometer, which you can pick up for the same price. See my recommendations below.
Don’t get me wrong, this is still a great unit and my top pick. And as far as analog refractometers go, this is as good as it gets.
Best digital refractometers for testing seawater salinity
Best digital refractometer for the money
|Measurement scale||PSU (0-50), PPT (0-150) SG (1.0000 – 1.114)|
|Accuracy||PSU +/- 2, PSU +/- 2, SG +/- 0.002|
The Milwaukee MA887 is a favorite with hobbyists and for good reason. It is easy to use and affordable, and you get many of the features found in salinity refractometer that are five times its price.
If you look closely, you might recognize the design. The MA887 refractometer is basically a rebranded Hanna HI96822 – the only difference is the price. You can buy the Milwaukee for a fraction of the cost.
A simple “no salt” calibration is all that is needed – a couple of drops of reverse osmosis/deionization (RO/DI) water, and you are on your way.
As for results, this dual-scale refractometer gives a measurement within three seconds and appears to be accurate enough. The LCD screen even displays the room temperature, a variable that can alter the results.
The only headache I had was when using the device in a bright room – I received an error message during testing. However, this was my fault, and the unit operated just fine when I turned the light off.
My only complaint is that the refractometer is pretty bare bones and doesn’t come with a case or pipette to easily measure out water. However, these can be purchased separately.
Best digital refractometer for accuracy – Misco
|Measurement scale||PSU (0-155) SG (1.0000 – 1.1180)|
|Accuracy||PSU +/- 1 SG +/- 0.0005|
Made right here in America, the MISCO AQUAR-H20 is the Rolls Royce of refractometers. It doesn’t get any more accurate than this, and best of all, it is designed to be used with aquarium water.
It is the most precise portable refractometer on the market – its nearest competitors are bench-top laboratory instruments that cost thousands of dollars. In comparison, this is a bargain.
In terms of operation, it is no different to any other digital refractometers. Simply calibrate the unit with distilled or deionized water, and you are good to go.
A rubber jacket will protect the device if you drop it – A necessary accessory, as this is an expensive refractometer to break.
As you see, a refractometer is one of the best ways to test the salinity of your aquarium.
And best of all, they start at an affordable price.
Do you use a refractometer for your aquarium? Let me know in the comments below!
Ian Sterling, founder of Fishlab.com, began his aquarium journey over 30 years ago, driven by a deep fascination for fish and their diverse personalities. His website, Fishlab.com, is dedicated to making fishkeeping accessible and enjoyable, offering beginner-friendly guidance, expert insights, and a community for aquarists to connect and share experiences.