How To Use A Hydrometer For A Saltwater Aquarium

If you’re breeding saltwater fish, you will need to measure the exact gravity and salinity of your aquarium on a weekly basis to maintain a habitable environment for your fish.

The Ultimate Guide To Saltwater Fish…

Normal salinity in a marine aquarium is around 0.03 percent and 0.05 percent. Normal specific gravity is around 1.02 and 1.024. An ocean hydrometer is the most effective way to measure both readings.

Using the Hydrometer –

  1. Rinse the hydrometer with clear water before filling it to make sure it is clean.
  2. Dip the hydrometer in your tank, and let it get filled completely.
  3. Take it out and place it on a level surface.
  4. Gently tap the side of the hydrometer with a pencil or plastic spoon to dislodge the bubbles.
  5. Watch the needle as it moves to the proper reading for the specific gravity and salinity.


Make sure you rinse your hydrometer with clear water again and dry properly before you put it away. Or else, mineral deposits will affect the next reading.

Saline deposits can pile up on the needle after a few uses. Place the hydrometer in a bowl of water deep enough to completely cover it, then add a tablespoon of vinegar to the water.

What is an aquarium hydrometer and how does it work?

A hydrometer is a device that measures the salinity in your aquarium, returning a result in either specific gravity (SG) or salinity PPT (parts per thousand.)

While there are two different types of hydrometers, which we will be looking at later, they both work similarly, and that’s by floating.

Simply put, the higher the salt content, the denser your water is. Because a hydrometer is a precisely weighted device, it will float in saltier water and sink in less salty water.

Keep in mind that you can’t go out and buy any type of hydrometer. Some are designed for testing beer while others are meant for testing honey or antifreeze. Obviously, these are not suitable for testing the salinity or specific gravity of your aquarium. So check properly before you buy it.

What are the different types of aquarium hydrometers?

When it comes to saltwater aquariums, there are two types of hydrometers that you will find, one much more popular than the other.

Swingarm hydrometers

These are the most common type of hydrometers for aquariums. Cheap and commonly available, you can get this on the shelves of most local fish stores.

Fill up the hydrometer by dipping it in your saltwater tank and then keep it on a level surface. The needle inside will rise according to the salinity of your aquarium water, which will give you a measurement. It’s this moving needle that gives the swing arm hydrometer its name.

Just line the needle up with the scale to determine the salinity of your aquarium.


  • Durable
  • Inexpensive
  • Easy-to-read


  • Requires strict steps to be accurate
  • Can be difficult to read
  • Large

Floating hydrometers

While it may look like a thermometer, but a floating hydrometer works differently. Just place the hydrometer in the saltwater aquarium, and it will float according to the salinity of your water.

Just how well this glass hydrometer floats shows if you have enough salt in your aquarium. If the hydrometer sinks, then you require more salt. If it floats too much, you need less. As far as testing your salinity goes, it’s effortless.

Just how well the hydrometer floats, is used to determine the specific gravity or salinity of your water.

So, why aren’t floating hydrometers more popular?

They are made from glass, so floating hydrometers are delicate. There’s a round design that rolls, making it easy to bump off your counter when cleaning your aquarium.

Then again, if you work with gloves, the slim design will be hard to grip. If this hydrometer falls, it will shatter into tiny pieces of glass.

Floating hydrometers are not very common in the aquarium hobby. In fact, in all my years of fishkeeping, I have only personally come across one aquarist who used them.

Thanks to the lack of popularity, floating hydrometers can be hard to find. There are quite a few aquarium brands that manufacture floating hydrometers, such as Sera and Imagitarium, but your local fish store probably won’t keep these on their shelves.

For this reason, aquarists who use a floating hydrometer usually buy them from stores that sell glass lab equipment.


  • More accurate
  • Smaller sized
  • Easy-to-read


  • Breakable
  • Expensive
  • Not readily available

How To test the hydrometers

To make the test fair, first, remove a few variables that could impact the results. After all, how can you pick the best hydrometer if your testing methodology is flawed?

You can’t. So to make sure you get the best results, you would need the help of some fancy scientific equipment.

First, it is worth mentioning that hydrometers are calibrated for a specific water temperature, 68-85°F (20-29°C).

So to make sure the water being tested remained a constant temperature, try using NIST-calibrated thermometers, the same ones used when testing the best heaters and best thermometers.

The water should be heated to 77°F (25°C) you can use the Eheim Jager TruTemp heater.

Next up, you should make sure that the salinity remains constant. After all, if you don’t know the salinity of the water, how could you determine the accuracy of the hydrometers tested?

So, you can use a good conductivity meter to check. Then cross-check the results with both a handheld and digital refractometer.

The best and most accurate hydrometers for your aquarium

If you are going to buy a hydrometer, then I assume you would want the absolute best.

Below, I will list out some of the best hydrometers for saltwater aquariums that money can buy. They have different features, and their prices vary, but they are world-class models all the same.

Consider these –

  • Kent Marine
  • Instant Ocean
  • Hikari Aquarium Solutions Accuprobe
  • Fluval
  • Coralife
  • Kordon


I trust this article has been helpful.

Good luck!

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