How Does Temperature Affect Salinity

How Does Temperature Affect Salinity?- How to Deal with it?

As an aquarist, you’ve always heard that change in temperature affects salinity. But as it is a tricky concept never actually understood the whole thing. 

So, you may want to know how does temperature affect salinity?

The salinity of the water is measured by specific gravity. This parameter is directly affected by temperature change. The change in temperature of water increases or decreases the density of the saltwater. This in turn changes the specific gravity. But there are solutions to tackle this change. 

Change of salinity can be harmful to the inhabitants of your tank. It’s better to tackle these changes as soon as possible. Thus, making your tank a safe place for different living organisms.

We have tried to cover all you need to know on this topic. We mentioned the ways to tackle the salinity change in your saltwater tank.  Let’s get to the depth of this topic!

Mythbusting: Does Temperature Actually Affect Salinity?

Salinity is the metric to measure the amount of salt in water. It is normally measured in ppt (parts per thousand) units. It refers to the number of salt particles that are in 1000 particles of water. Let’s consider your tank’s salinity is 35ppt. It means there are 35 salt particles in 1000 water particles. 

If you don’t add any salt externally, the amount of salt stays the same. So, what actually happens?

The change occurs in the specific gravity. Specific gravity decreases with an increase in temperature. And Specific gravity increases with a decrease in temperature. This in turn changes the salinity

So, comparing two samples with different temperatures, warmer ones will have less salinity. On the other hand, the salinity of the colder samples will be greater. 

Temperature and Salinity Relationship

As we know now, salinity is measured from the specific gravity of saltwater. 

Specific gravity is also referred to as relative density. From the definition, it is the ratio of a sample of seawater at a specific temperature to the density of fresh water at a specific temperature.

When the temperature of saltwater rises, the water molecules spread apart. As a result, it occupies more space and becomes less dense. In the meantime, salt remains the same. Therefore, the decrease in salinity and decrease in specific gravity occurs.

In contrast, when temperature decreases the water molecules shrink. The density of water keeps increasing and the salt stays the same. As a result the specific gravity and salinity increase.

Let’s check an example of how specific gravity decreases with a rise in temperature.

To find out the result I kept the salinity fixed at 30.6ppt. After increasing temperature at frequent intervals, I took readings with this refractometer that you can buy from Amazon. The result was the same as I anticipated.

Temperature in °C/°FSpecific Gravity with Salinity at 30.6ppt
7°C (44.6°F)1.025
13°C (55.4°F)1.024
18°C (64.4°F)1.023
22°C (71.6°F)1.022
25°C (77°F)1.021
28°C (82.4°F)1.020
31°C (87.8°F)1.019
34°C (93.2°F)1.018
37°C (98.6°F)1.017

You can see here a gradual decrease in specific gravity with increasing temperature. All these numbers look a bit confusing, right? We have also explained it graphically to make it easier to understand.

As you can see the line is descending as temperature increases. Therefore, with the rise in temperature specific gravity keeps on decreasing.

Effect of Evaporation

But, there’s a catch! Temperature directly affects salinity levels. At higher temperature water molecules starts to turn into vapor, this is called evaporation.

The degree of evaporation increases with the increase in temperature. Because the molecules move faster when heated. However, in comparatively lower temperatures evaporation can occur too.

That’s why you must monitor the temperature with thermometers regularly in your tank. Otherwise, plants like frogbits can die of increased temperature.

You can check out these digital thermometers for your aquarium. I personally used them in my saltwater and freshwater tanks.

However, evaporation can occur in lower temperatures as well. Through evaporation, the water inside the aquarium slowly becomes water vapor. 

Separation by evaporation : Tigtag

Source: TigTag Aksorn

As the amount of water reduces, the amount of salt remains unchanged. This increases the amount of salt in the reduced amount of water. Thus, increasing the level of salinity.

But how to tackle all these changes and yet not harm the tank’s population?

How to Tackle the Salinity Change

Coping with salinity change is not rocket science. It’s quite easy and requires a little patience. As the temperature changes the salinity, it can also be used to control it. But here’s the thing, it is not the most efficient process.

The salinity of the tank may increase or decrease. However, to keep the salinity at optimum, you should try changing the water. Keep in mind, don’t try to change everything overnight.

Sudden change in water will come as a shock to the fish. Take your time to change the water in your tank. Otherwise, you’ll share the same ill fate as me. When I started out with this hobby I was impatient and all of my fish died. 

My whole tank’s population was in jeopardy because I wanted to change the salinity quickly. So, let’s take a look at how you can successfully change tank’s salinity.

My Tank Salinity has Increased

The concept of salinity in seawater is quite different from that of your aquarium. Evaporation plays an important role there. Here’s the deal, for evaporation the salinity increases with an increase in temperature. Moreover, an increase in temperature can be a reason behind your java fern not surviving.

And, decrease in temperature causes a dip in salinity. That is why hobbyists like us assume an average salinity level for ocean water. It is around 1.026 specific gravity with 35ppt. 

But what can you do when your tank’s salinity exceeds this threshold?

Does Changing the Temperature Reduce Salinity?

You might want to increase the temperature to change the specific gravity. However, it contradicts the effect of evaporation on salinity.

Because evaporating the saltwater will increase the salinity of your precious tank. Also, it will take a huge amount of heat to do that. This huge amount of heat might even kill the frog in your tank.

Letting the temperature rise that high can harm your plants, fish and corals. It is also one of the reasons behind corals shutting off.

So, it is quite impractical to try and change the salinity by changing temperature. However, keeping an average temperature is a must to keep the salinity optimum.

The average temperature varies according to your tank. Your saltwater tank will not require the same temperature as your freshwater tank.

As you can see, it’s different for different types of aquariums. And that’s where the debate between aqueon and marineland comes in.

However, to control the temperature you can use these reliable and easy-to-control aquarium heaters. 

You should pick your aquarium heater according to your tank volume. 

Does Adding More Water Reduce Salinity? 

Yes, adding more freshwater will reduce the salinity level significantly. Basically, you can try to reduce the salinity of your water tank by two methods. 

Method 1: Manually Replacing Saltwater with Freshwater

Firstly, pick up 10% of tank water from your tank. Then add the same amount of freshwater to it. Make sure you use reverse osmosis deionized water to fill up the tank. 

This RODI water is the purest form of water with 0 percent of ions in it. It’s water in its purest form. Also, remember to adjust the temperature of the water to be added. It should be the same as the tank water.

Now use your refractometer or hydrometer to check the salinity of your tank water. Keep repeating this process until it reaches the optimum level.

Method 2: Using Auto Top Off System

Manually repeating the water-replacing process can be tiresome. It is also time-consuming and boring as well. Let’s face it, most of us don’t want to go through all of that hassle.

The good news is there is a machine that automatically helps you in this process. It is called Auto Top Off system, commonly known as ATO. 

It’s a machine to feed your tank freshwater gradually. It innately processes the data of your tank and adds freshwater needed in your tank. All you need to do is keep the ATO reservoir filled up with RODI water.

Trust me when I say this, you won’t have to ever worry about salinity rising in your tank. I’ve used some great ATO myself and here are some suggestions for you.

You can get any one of them and watch your tank flourish.

My Tank Salinity has Decreased

The main issue many rookie saltwater hobbyists face is to keep stable parameters. I wasn’t an exception either. I’ve made numerous mistakes to learn what is best for my tank.

Even if you have all the equipment, all you actually need is effort. The more effort you give while maintaining your tank the better. However, regardless of the effort, unexpected things may occur.

Like the reduction of salinity in your tank. To increase the salinity level you need to add more salt. So how can you measure how much salt do you need?

Firstly, you need to check the salinity or specific gravity of your saltwater. It’ll help you to calculate the amount of salt you need to restore balance. You can use this automatic salt addition calculator to calculate the required amount of salt.

Another way is to calculate it manually. Don’t worry I’ll help you out with the maths.

Let’s say you have 30ppt of salinity in a 300-gallon tank. This means there are 30 grams of salt per liter. Now you need to reach 35ppt.

If you want to increase 1L of 30ppt to 35ppt you’ll need 5g salt.

Now, 1 gallon = 3.8L, so it’ll take (3.8*5) =19g per gallon. Again, for 300 gallon you’ll need (300*19) = 5700 grams = 5.7 kg of salt. 

This is not exactly correct but you get approximate information about the required salt. 

Then putting in the salt is the most sensible thing right? Well, let me clear something first.

Can You Directly Put Salt in a Tank?

You never should add salt directly in your tank. This will cause a huge imbalance of tank water. It will disrupt the regular functionality of your tank. It will also put the life of the tank’s inhabitants in danger.

If you are starting out a new tank without any living organism, then it’s fine. You can add salt to that tank. However, never add salt directly into a populated tank. It will be suicidal.

So, what is the answer to this problem, how to add salt in aquarium?

First, take out 10 percent of aquarium water. Then pour the same amount of RODI water into a large bucket. After that, dissolve the calculated amount of salt in the RODI water. Next, match the temperature of it with the aquarium water. Finally, top off the tank with the salt-water mixture 1/2 gal each time. Keep doing this till it reaches the desired salinity level.

Different types of tanks require different types of salt. There are specifically formulated salts in the market. You can check them out here and get one for your precious tank.

No one knows your tank better than you do. So, be wise when choosing the salt type. Because it will make or break your precious aquarium system.

Are You Sure That the Solution Worked?

We have gone through various types of solutions in this article. Now you know about how to lower your tank’s salinity and also how to increase it. But how can you be so sure that it worked perfectly?

That’s where measuring salinity and specific gravity comes in. In earlier days, hobbyists used to check these parameters by measuring with a hydrometer. However, they are very unreliable. 

The reason behind using a hydrometer, the cost of a refractometer. The good news is nowadays it has become affordable. The increment in accuracy while using a refractometer is much better than using a hydrometer. Here are some reliable refractometers that I thought gave a great performance.

So, how to use a refractometer

Using Refractometer to Check Salinity [ Step-by-Step Guide]

Using a refractometer is not rocket science. It’s nothing like making pond in terrarium. You’ll have to calibrate the refractometer first then measure the salinity of your tank. Here is a step-by-step guide to help you out.

Step 1 of 6: Dripping the Main Prism with RO Water

First, put some drops of RODI water on top of the main prism. This can be done by opening the daylight plate. Instead of RODI water, distilled waterworks as well.

Now, close the plate and let the water soak the entire surface of the main prism. You have to make sure that there are no dry spots or bubbles on the surface.

Step 2 of 6: Holding the Refractometer Towards Light Source

Once you’re done soaking the main prism, keep it there for 45 seconds. Then you should hold the instrument towards the light source. Next, if you look using the eyepiece, you’ll see a circular field. If the visual is not clear then try adjusting the focus.

The color of the circular field should be blue. Blue circular field means salinity is 0. If you don’t see something other than blue you need to calibrate the refractometer.

Step 3 of 6: Calibrating Using Calibration Screw

To calibrate the refractometer, you need to look across the eyepiece. Then you need to adjust the calibration screw. Adjust it until the blue field and the white field converge precisely at 0.

Step 4 of 6: Soaking the Main Prism with Salt Water

Now your refractometer is properly calibrated, it’s time to measure the Salinity. To do that, first, put some tank water on the prism using a pipette. Now, wait approximately 45 seconds for the sample to adjust with ambient temperature.

Step 5 of 6: Observing Through the Eyepiece 

This step is almost similar to the 2nd step. You need to observe through the eyepiece to check  the readings. Notice the lower boundary of the blue portion and the upper boundary of the white one. The difference between these two portions represents the salinity level of your saltwater.

Step 6 of 6: Cleaning the Main Prism

Finally, you need to clean the main prism with distilled water. You can use RODI water as well to clean the prism. Next, wipe clean the water with a soft cloth. You need to make sure that it’s totally free of water particles.

Maintaining stability of your tank’s salinity is very crucial. It either can make or break the whole ecosystem inside the tank. It’s always better to prevent something disastrous before happening. 

And that’s why here are some tips on how can we prevent salinity change.

Tip 1: Top off with Fresh Water Only

One thing that we keep forgetting is that the water evaporates, not the salt. The salinity increases as the water volume reduce through evaporation. To lower the salinity you must replace the aquarium water with fresh water.

If you pour in saltwater to reimburse for evaporation, the salinity will rise. Because you have added more salt into the existing saltwater. In contrast, if we use fresh water to fill the tank it’ll keep the salinity stable.

On the other hand, replacing the evaporated water with an equal volume of purified fresh water will keep the salinity right where you want it.

Another thing to remember is the time between the top-offs. It should be done swiftly to avoid salinity fluctuations. We recommend considering this process as one of the daily chores.

Tip 2: Matching Salinity of the Added Salt Water

Now what precautions can you take while adding salt water? Well, you need to compare the salinity of the water to be added with tank water. 

Here’s a small tip- don’t trust the salinity level right after you’re done mixing new saltwater. Let the solution sit overnight. Once you’ve heated the newly mixed saltwater, then check it.

There might be a slight difference in salinity measurement from before. Because by then the salt will have the chance to get mixed and stabilized. If there are differences then you should add either salt or freshwater as required. 

Now it’s ready to be poured down slowly into the tank.

Tip 3: Regular Checking of Temperature and Adjusting Temperature

Regularly checking for any abnormality in your tank’s temperature might sound easy. But it takes practice and perseverance to execute it. 

To prevent any unimaginable circumstances, routinely checking the tank’s temperature is a must. If there are any discrepancies from the expected value adjust your aquarium heater accordingly.


What is the optimum salinity range for fish?

Most of the fishes prefer to be in an optimum salinity range. It’s around 1.023 to 1.028. It does not mean that it’s safe for the fish inside this range. The best salinity level for fishes is 1.026 ± 0.001.

What happens to corals if salinity is too high?

If the concentration of dissolved salt is higher then it’s better for coral growth. A Salinity level above 1.027 will have detrimental effects on corals but won’t cause death. However, anything above 1.030 is highly dangerous and you should try to reduce it immediately.

Is 1.030 Sp. Gravity is too much?

A saltwater aquarium cannot handle salinity levels as high as 1.030 Sp.gravity. It would be best to maintain the salinity around 1.026. Otherwise, it’ll be harmful to corals and fish. 


Salinity is a very important parameter for your saltwater tank. It is deeply related to the change in temperature. There are many confusions regarding this relationship. However, we tried to untangle this confusion once and for all.

That’s all from our part. We’ve tried our level best to explain how does temperature affect salinity. 

If you still have any more confusion do comment below. Good luck!

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