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Last update: April 7, 2021

Refractometers – The BEST Way to Test the Salinity of Your Saltwater Aquarium

So, you have decided to set up a saltwater tank?

Congratulations!

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…

Example of refraction – lines through a drinking glass

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.[1]

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)

Analog handheld refractometer with eyepiece to measure aquarium water salinity

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 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 refractometers display salinity measurements in parts per thousand (PPT) and specific gravity (SG).

Pros

  • Affordable
  • Doesn’t need batteries
  • Portable

Cons

  • Cheaper models are less accurate
  • Requires more effort to use
  • Manual calibration
  • Subjective measurements
  • Needs a bright light source

2. Digital salinity refractometers

Digital refractometer to test aquarium salinity

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.

Pros

  • Accurate
  • Easy to use
  • Quick

Cons

  • Expensive
  • Require batteries
  • Bright light can cause errors

Which are the best refractometers?

Best handheld refractometer for the money – Ade Advanced Optics

Ade optical handheld refractometer for measuring salinity

Model number SSA0010
Measurement scale Salinity PPT (0-100%), SG 1.000 – 1.070
Accuracy Salinity PPT +/- 2.0%, SG +/- 0.1
Warranty Two years

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

Vee Gee premium handheld seawater refractometer for aquarium salinity STX-3

Model number STX-3
Measurement scale Salinity PPT (0-100%), SG 1.000 – 1.070
Accuracy Salinity PPT +/- 1.0%, SG +/- 0.001
Warranty One year

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

Milwaukee MA887 digital salinity refractometer for aquariums and seawater

Model number MA887
Measurement scale PSU (0-50), PPT (0-150) SG (1.0000 – 1.114)
Accuracy PSU +/- 2, PSU +/- 2, SG +/- 0.002
Warranty Two years
Instruction manual PDF

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 refractometers 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 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

Misco AQUAR H20 palm digital salinity refractometer for aquariums and salt water

Model number H20
Measurement scale PSU (0-155) SG (1.0000 – 1.1180)
Accuracy PSU +/- 1 SG +/- 0.0005
Warranty One year
Instruction manual PDF

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.

If this is well in your budget, check out the H50 model an upgrade of the H20 that also tests the chlorinity and conductivity of your fish tank.

Conclusion

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

I've been keeping fish for over 30 years and currently have 4 different aquariums – it's an addiction. I'm here to teach you everything there is to know about fishkeeping.

I also use this site as an excuse to spend lots of money on testing and reviewing different aquarium products! You can find my reviews here.

Comments (10)

Hello,

I’m looking for a refractometer and came across this post. Thank you very much. I was originally looking at the manual models simply because they are less expensive however I started to look at the digital models after reading your post. I came across these pen types. I found a few types like this. They all say they’re for testing saltwater aquariums (among other salty things). Have you seen or had any experience with these? The price is perfect (almost seems too good to be true) but I would rather buy a similarly priced manual model if these are garbage.

Thanks again for your post!

Karen

Hi Karen,

I have no experience with the pen-style refractometers. However, given their size and cost I would have some concerns as to their accuracy – all of the accurate digital refractometers I have used are bulky in their size.

Thanks for this Ian! Hopefully Rob replies as to the pen type he purchased. These are so sleek (and digital) that they would be a welcome addition to the hobby if they work!

BTW: I have read a lot of your posts. I implemented the bleed valve for air pumps you wrote about. It never crossed my mind that just having a control valve on an airline can wear out an air pump over time.

Keep writing those articles. They’re great!!!

Oops, I already replied to your comment below Robs! If I can test one and it works just as well I will happily update the guide to include it. Having a bridge between the affordable but somewhat clumsy manual refractometers and the higher-end ones would be awesome.

I’m glad the bleed-valve advice helped you! I learned it the hard way years ago. I used to buy pumps that were too powerful and constantly blew the diaphragm trying to regulate the air pressure with an in-line aircontrol valve. At the time, I thought it was because the air pumps were poorly made, but now I know better.

The “pen” types are great! They are SPOT ON. I bought a Milwaukee tester. Not so much. There’s no need to complicate the use of a prism by having it read electronically for 5x the price.

Hi Rob,

Like all test equipment, you can get good equipment and bad ones. It’s a generalization to say “pen types” are spot on. There are plenty of low quality ones floating around. However, I’m glad you have found something that works for you!

Hello Rob. Thank you for the reply! I’m glad to hear your has been good with the pen types! Can you tell me which pen type you bought? I happened to come across the pen types on Amazon while looking at the manual ones. Some had no reviews but one of them had a couple reviews and they were all positive (4 to 5 stars). It is the one sold by Gain Express (but that’s on Amazon Canada). I found one that looks identical with the same rating (4 to 5 stars) on Amazon US sold by TEKCOPLUS. Having said that I would like to know which one you bought as that would probably be the safest. I agree with Ian that all products have good manufacturers and bad manufacturers so life would be easier to get one that someone already swears by as being great. 🙂

I’m Curious too Karen. I hope he follows up with which brand he uses (hopefully he is located here in the USA, so we can both easily get one!)

I’m just in the process of setting up my first saltwater tank (getting the equipment/tools. etc). If Rob doesn’t reply in the next few weeks then I will just take the plunge and get one of the models I noted above and will let you know how well it works. I have a friend with one of the manual types so I can borrow that to compare the readings. 🙂 Fingers crossed that these work so there will be another option available to do salinity testing. 🙂

How exciting, saltwater tanks really are something else. They are a lot of work though, but the fish you can keep are absolutely stunning. That would be really awesome if you could road-test the pen type refractometer! I hope your set up goes smoothly. While cycling is time consuming, it’s well worth it in that you’ll have fewer problems arise.

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