How To Use A Refractometer For A Saltwater Aquarium

For those who are new to the word, a refractometer is a tool that helps determine the exact salt levels in an aquarium.

The Ultimate Guide To Saltwater Fish…

Refraction is the change in the direction of light as it beams through the water. Refractometers measure the extent to which light in an aquarium changes direction.

It works by taking the refraction angles and correlating them to an already established refractive index (n) value. With these given values, you can determine the concentration of solutions.

Basically, solutions have varying refractive indexes, which are all dependent on their concentration in water.

Which Refractometers are Suitable for Aquariums?

Not all refractometers are suitable for your tank, in fact, it is common to see many hobbyists use the wrong ones for saltwater testing.

Most salinity refractometers are built to gauge the amount of sodium chloride in water, however, saltwater contains other elements like magnesium and calcium.

And most salinity refractometers are not calibrated for these extra elements. What this means is that an average refractometer is prone to as much as 5% errors when measuring salinity.

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.

If you’re one of those who want the most accurate measurements, then be advised to buy a seawater refractometer, not a saltwater refractometer.

What are the Different Types of Refractometers?

There are many models of refractometers in the market. Many of which can test a wide range of fluids, such as blood, urine, beer, and honey.

They may all have similar physical features, but they are not all the same, as far as functionality is concerned.

For this reason, you have to be sure that the one you buy is meant for measuring salinity. If you don’t, you would be frustrated by two things – getting the wrong measurements and wasting your money.

When it comes to fishkeeping, there are two types of refractometers that are commonly used. These are –

Handheld Salinity Refractometers (analog)

This is the cheapest and most popularly used aquarium refractometer you can find.

Here’s how it works –

Simply calibrate the refractometer, add a few drops of aquarium water to the refractive prism and look into the eyepiece to get the salinity reading.

While cheaper models will not be as precise as the digital models, an analog refractometer will be accurate enough for the majority of aquarists.

I recommend choosing a handheld refractometer with automatic temperature compensation (ATC), which gives you a more accurate measurement.

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

They have their pros as well as their cons


  • Affordable
  • Doesn’t need batteries
  • Portable


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

Digital Salinity Refractometers

Comparing a digital refractometer to a handheld one is similar to comparing a horse and carriage to a car. A digital refractometer does the same thing but makes the entire process much simpler.

Digital refractometers are easy to use and return a precise salinity measurement in a matter of seconds.

The measurement is clearly displayed on the LCD screen, so you do not have to interpret the results yourself.

As you can imagine, all these great features come at a cost, and digital refractometers are more expensive than the handheld variety.

Below are its pros and cons


  • Accurate
  • Easy to use
  • Quick


  • Expensive
  • Require batteries
  • Bright light can cause errors

Which are the Best Refractometers?

I’ll list a few recommendations below.

Ade Advanced Optics

Having a built-in automatic temperature compensation (ATC) and extra accessories like cleaning cloth, pipette, and storage case, this refractometer gives you incredible value for your money.

If you are new to the hobby and just want a reliable refractometer at a budget price, then this is a good choice.

Vee Gee Analog Refractometer

Vee Gee’s analog refractometer works just the same as the cheaper models but is much more accurate.  The build quality is excellent, rugged, and durable.

Mediavine Refractometer

The major issue with this model is that they are expensive. And unless you like to do things the old-fashioned way, I advise buying a good digital refractometer, which you can get for the same price.

Don’t get me wrong, this is still an ideal unit and my top pick. And as far as analog refractometers go, this is as good as you can get.

Milwaukee MA887 Refractometer

This is a favorite with hobbyists, and for obvious reasons. It is simple to use and affordable, and it offers many of the features found in refractometers that are way more expensive.

If you look closely, you might recognize the design. The MA887 refractometer is sort of a rebranded Hanna HI96822, the major difference being the price.

However, You can buy the Milwaukee for a fraction of the cost.

A simple “no salt” calibration is all that is needed – a few drops of reverse osmosis/deionization (RO/DI) water, and you are good to go.

As for results, this refractometer produces a measurement in a space of 3 seconds and seems to be accurate enough. The LCD screen also shows the room temperature, and this is a variable that can alter the results.

An issue you may have is when using the device in a bright room. You will receive an error message during testing.

Another complaint is that the refractometer is very bare, as it doesn’t come with a case or pipette to easily measure out water. However, these can be bought separately.


I trust this article on how to use a refractometer for a saltwater aquarium has been helpful, as well as the other information provided here.

Take care!

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