How to Measure Salinity in a Saltwater Aquarium using a refractometer.

how to use a refractometer for salt levels in saltwater tank.

In this post I’ll show you how to measure the salt level in a Saltwater Aquarium, also known as salinity, using a refractometer.

I have a YouTube channel on saltwater aquarium care called Rotter Tube Reef, which at the time of this writing has over 11,000 YouTube subscribers.

Salinity (salt) is the most fundamental parameter in a saltwater aquarium. Without salinity, it isn’t saltwater. Because salinity is such an important parameter, it is often overlooked.

Salinity is the measure of dissolved salt content in water. Salinity is typically measured in parts per thousand (ppt) or specific gravity (sg). The more salt in your water, the higher your readings will be. Saltwater aquarium salinity is usually kept between 1.020-1.028sg. Some saltwater aquarium keepers like to keep fish only aquariums at lower levels (1.017+) to keep salt costs low.

As a side note, it has been said by some that lower salt levels will kill parasites in your saltwater tank. THIS IS COMPLETELY FALSE. Only FRESH water will kill saltwater parasites such as ich and marine velvet. Saltwater fish cannot be kept in fresh water for long, but a minute or two is ok.

The lower the salt level in your saltwater tank means there are higher levels of oxygen in your water. The higher the salt level in your saltwater tank means there are lower levels of oxygen in your water.

how to measure salt in saltwater tank with refractometer

If you have a fish only saltwater tank, it is best to keep salinity levels lower, such as at 1.017 or 1.018. You can measure salt levels in your salt water tank using a refractometer.

Many reef aquarium keepers like to maintain salinity at higher levels, sometimes as high as 1.031 because salinity tends to be higher around a reef. A reef has coral and coral thrive with higher salt levels at least 1.028. Choose what is right for your aquarium and make sure it stays stable.

A good refractometer is a precision instrument for measuring salinity. Without getting technical, a refractometer works by measuring how much light “bends” as it passes through the water. As the salinity in the water changes, so does the angle of the refraction, or “bend.” Be sure you are buying a refractometer made for measuring salinity in a saltwater aquarium. Refractometers can be used to measure any number of things, and are calibrated depending on what they were manufactured for.

When you buy a refractometer, and periodically afterwards, you should check the calibration of your instrument. You can use salinity calibration fluid,  or you can use distilled or RO/DI water, which should measure 1.000sg.

how to measure salt in saltwater tank with a refractometer
Salinity measuring just under 1.020

To use a refractometer, place a few drops on the glass plane and then place the clear cover over the sample. Point the refractometer at a bright light source, like your tank. Look through the eyepiece in the back to get the measurement. Your salinity is wherever the colored half of the viewfinder stops.

Salinity is a key parameter to measure for in a saltwater aquarium. Salinity should be one of the first and most frequent tests you do on your saltwater tanks.

Just as water evaporates from the sidewalk after the rain stops, the same goes for your aquarium. Water will evaporate and must be replaced. REPLACE EVAPORATING WATER WITH WATER ONLY! NEVER REPLACE EVAPORATING WATER WITH SALTWATER!

THE ONLY TIME YOU ADD SALT TO YOUR SALTWATER TANK IS WHEN YOU DO A WATER CHANGE AND ARE REMOVING SALTWATER FROM YOUR TANK. If you remove 10 gallons of saltwater, you replace 10 gallons of saltwater.

Water evaporation is only water loss, leaving the salt (a solid) in the tank.

Water evaporation in your saltwater tank results in the salinity level being higher. This makes sense because with less water, there is a higher salt level.

With more fresh water, the salt level is lower, since the salt level is “watered down.”

I replace a gallon of fresh water daily in my 125 gallon saltwater tank due to water evaporation. Some people have purchased auto top off units but I prefer to manually add water daily. It’s no big deal. I add water when I feed the fish each night.

Now that you know how to keep the salt level (salinity) constant in your saltwater aquarium, your saltwater fish and corals will thrive!

You can purchase this Refractometer and read the Amazon reviews HERE

Check out my YouTube channel on saltwater aquarium care: ROTTER TUBE REEF

Thanks for reading! Follow my blog here.


KILL GREEN HAIR ALGAE with FOOD GRADE 12% HYDROGEN PEROXIDE. Dose 3ml per 10 gallons every other day.

Finnex LED Aquarium light:
MAG-FLOAT glass aquarium cleaner:
REFRACTOMETER to measure salt level:
Carib Sea Arag-Alive Sand:
100% Pure Magnesium:
SYPHON to vacuum sandbed:
Ecotech Marine MP40W:
Measure Master Syringe:
Turkey baster for cleaning live rock:
saltwater tank maintenance gloves:
Seachem Cupramine Copper to kill ich:
Copper Test Kit:
Miracle Mud (Marine) 5 lb:
Red Sea Coral Pro Salt:
Aquarium Filter Media:
3ft X 1 Flex Hose by Eshopps:
Instant Ocean Sea Salt:
Rio Plus 2500 return pump:
New Life Spectrum fish food:
Stainless Steel Algae Scraper:
Blue Yeti USB Microphone:
EHEIM Jager Heater:
Seachem Ammonia Alert:
Tripp Lite 7 Outlet Surge Protector:
Two Little Fishies Seaweed food:
My favorite Protein Skimmer:
Marineland Canister Filter:

My eBook is available on Amazon:
How to kill ich and marine velvet parasites in your saltwater aquarium FOREVER.


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I’m a video editor of 25 years, audio engineer 20 years, photographer 25 years and guitarist 20 years. I post photography and stop motion animation videos, but mainly guitar playing.