What does aquarium salt do for freshwater tanks?

If there’s one topic guaranteed to stir up controversy in the world of the hobby aquarist, it’s this: what does aquarium salt do for freshwater tanks?
Aquarists practically square off at dawn with pistols on this issue.

It’s polarising – any online search will provide you with plenty of articles declaring that YES, salt is ESSENTIAL for freshwater tanks, and listing all the reasons.
It will also provide you with just as many emphatically declaring the opposite.
NO, salt ISN’T necessary for freshwater aquariums, with just as many reasons against.
Arguments vary on online forums between those on each side of the issue. It’s clearly a topic that stirs up strong feelings in the aquarist world.

In this article, we’ll look at all the arguments for and against using salt in your freshwater aquarium.
From therapeutic use to recommended concentrations, health benefits to potential dangers, we’ll cover all the pros and cons of adding salt to your freshwater tank.

Let’s start with the basics: what is aquarium salt?

Aquarium salt is a general term that covers a range of salts you can add to the water in your freshwater aquarium.
Standard sodium chloride, known as NaCl, is just one of them.
So when is a salt not a salt? Are all salts created equal?

There are a range of different elements that are collectively called “salts”. Some of these include other minerals, such as:

They can also include different compounds, such as:

You don’t need to have a science degree to understand the mineral make-up of aquarium salts, however.
There are three very common kinds of aquarium “salts” that you might buy to use in your fishkeeping practice.

Also known as “tonic salt”, this is very different to the table salt that you have in the kitchen, which should generally never be added to an aquarium (more about that later).

Freshwater aquarium salt is made from evaporated sea water.
It’s specially designed for freshwater aquariums, so it doesn’t have any additives such as iodine or anti-caking agents such as calcium silicate.
It’s easily available online and from your local fish or pet shop.

These salts are pure sodium chloride (NaCl).
There are also no extra ingredients added to these types of salt, which makes them a popular favourite for salting advocates who like to add it to freshwater tanks.

Marine salt is an altogether different beast.
It’s a special salt that is much more complex, and it’s created with a range of different blended minerals such as magnesium and calcium.
These minerals can improve the quality of water for marine invertebrates and fish.

This is the 64-million-dollar question.
Those who argue for salting say that keeping your freshwater tank lightly salted can improve the health of your fish, act as a preventative agent against various kinds of parasitic infections, and help to cure various diseases.

Adding salt is said to make life easier for your fish, by helping to keep them stress-free, reducing osmotic pressure in the water, inhibiting the uptake of toxic nitrates, supporting the production of their protective slime coat and helping to heal wounds more quickly.

Osmoregulation is one of the most common arguments that comes up in relation to adding salt to your freshwater aquarium.

Salt does have a direct relationship to the water pressure in your aquarium.
If you remember your high school science lessons, you probably learned about osmosis – where salts and fluids can move through a semi-permeable membrane from a strong concentration to a weaker one.

This is nature’s way of trying to balance the concentration of salts and electrolytes in the water.
Your freshwater fish are naturally more salty than the water that they live in, and their skin is semi-permeable.
This means that by the process of osmoregulation, fish constantly discharge body salts into their tank water, and take tank water into their bodies.

This is osmosis at work, where your aquarium water is trying to equalise the salinity of your fish with the salinity of your aquarium.
To balance the process of osmosis, your fish need to reabsorb salt from their tank as they release a vast amounts through urination. Some freshwater fish can produce their own body weight in urine in just three or four days.

This process is constant and the salinity level in the tank changes constantly, though usually at the micro-level as the quantities are so small relative to the overall volume of your tank.
However, if your fish don’t absorb enough electrolytes from the water – for example, if they’re under stress – they can go into what’s known as osmotic shock.
Osmotic stress can be a common side-effect of transport, for example, when you’re bringing fish home from the pet shop.

You can compare it to the process of giving a patient in hospital a saline drip.

Another argument is that salt can help to prevent nitrate poisoning in your tank.

Nitrate levels can spike if your tank doesn’t have sufficient beneficial bacteria to absorb this natural but highly toxic by-product of your fishes’ excretions.
That can happen if your tank is new, or if your filter suddenly stops working.

By acting quickly and adding salt at a level of 30x the concentration of nitrate in the water, you can create a natural barrier where the chloride ions will prevent your fish from absorbing the nitrates.
This can give you time to get a new filter installed quickly to take better care of the issue.
However, others advise that the level should be even lower, and that 100mg (0.1g) salt per litre is adequate, as it’s a very low salt level that can be tolerated by nearly all kinds of freshwater fish.

A salt bath can quickly and efficiently remove some kinds of external parasites from your fish.

It disrupts the salt/water balance in the cells of harmful organisms, dehydrating them until they let go.
These include protozoa (one-celled parasites) on skin, gills and fins, including Chilodonella, Trichodina and Costia.
The bath can also help to stimulate your fish’s protective slime coat, which helps them to fight off disease.

A salt bath can also quickly get rid of fungal growths on your fish – easily recognisable as wispy white “fluff” attached to your fish.
Lastly, a salt bath can help to clear excess mucus from the gills of your fish – but can also cause mucus to build up!

Again, this is a dividing issue.

Remember that these are freshwater fish you’re dealing with, and too much salt can kill them.

No clear guidelines exist here. Practitioners recommend ranges from one teaspoon per 300 gallon to 1 tablespoon per 500 gallons.
Others say you should add about 0.3%, or 12g per gallon (3g per liter). This equates to a dose of around 2 level teaspoons per gallon.

in this situation, the chloride ions need to be 30 times the concentration of the nitrate ions in the water.
Run a water test to determine your nitrate levels in the tank, and adjust accordingly.
Nitrate levels become toxic at about 0.1 parts per million (ppm), so that would require about 3ppm of chloride ions per volume for correction.
A good guide is that one teaspoon of salt would be more than sufficient for a 300-gallon tank.

In this situation, concentration levels should be around 4 teaspoons per gallon in the water that you use for the bath (not inside the aquarium!). Opinions differ around how long the salt bath should last.
Some say it shouldn’t be more than 30 minutes.
Others argue that baths can last anywhere from 55 minutes to 2 hours .
Tricodina, Ichthybodo and Epitylis all respond well to salt baths.
Some aquarists recommend you add 2.5 cups of salt for every 10 gallons of water, but make sure the fish are only in the bath for 5-10 minutes.

Here is where the water gets murky, so to speak.
Opinions are firmly divided on this issue but there are three major schools of thought:

As long as you are keeping up good hygiene, water changes and there is adequate good bacteria to ensure you have clean water, freshwater fish don’t require any additional salt or mineral elements in order to live happily and healthily.

The additional salt can support your fish with their osmotic regulation and stress levels, making their lives easier.

Salt can cure some parasitic infections and fungal growths, but it should be used as a tonic only in those situations, and only in a bath outside the aquarium.

Some fish, like cichlids or goldfish, can respond nicely to a little salt in their freshwater aquarium or to a medicinal bath.
For goldfish particularly, salt can be a good way to cure some of their common ailments.

However, there are some fish that don’t respond well at all to salt.
Some barbs, South American tetras, corydoras, livebearers, catfish and koi are well-known for their salt intolerance.
A high salt concentration can make them lose their equilibrium and they will simply roll over.
If you don’t act fast at this point your fish may well die.
But opinions vary, and some research shows that even supposedly salt intolerant fish can adjust to very small levels of salt in their water.

Some invertebrates are very sensitive to salt, such as snails.
If you’re using zeolite to reduce the ammonia levels in your freshwater tank, the effect can be reversed during a salt treatment.
So make sure you remove all invertebrates and any zeolite before using salt.

Studies have shown that, as you’d expect, fry aren’t as tolerant of salt as larger, mature fish.
Research conducted on blue gouramis, black widow tetras, Buenos Aires tetras and zebra danios show they can tolerate 1 part per thousand (ppt) [1ppt= 1g per litre] but that they start to suffer at higher dosages.

The short answer here is that it depends on the kind of plant!

Algae hates salt, for example, basically dying as soon as it meets salt.
So, a salt treatment can very effectively eliminate all traces of algae from your freshwater tank.

However, most freshwater aquarium plants don’t respond well to salted water.
They’re freshwater, after all. Salting the water means the plant will begin to dry out.
While some species are more sensitive than others, the better rule of thumb is that you shouldn’t add salt if you have any live plants in your tank.
The lethal point for plants comes into effect at about 1000 micrograms per litre.
One teaspoon equals approximately 5500 mcg.

This is a thorny question, as the different schools of thought advocate for both yes and no.
The truth is that we simply don’t know how fish respond to even very low levels of salt in an aquarium.
Does it irritate their eyes constantly, the way chlorine does for us when we swim in a swimming pool?

We do know that freshwater fish cannot live long-term in salt water.
And many fish health experts don’t recommend that you keep your water salted.

If you do decide to go ahead and use salt, it’s important to know how to add it.
Don’t just throw it in! Be mindful of the concentration levels, dissolve first, add a little at a time, don’t place it onto plants, etc.
There are few things to take into consideration.

If you decide to lightly salt your fishtank, add it slowly.
Try half the needed dose one day and half 48 hours later.

The only way to get salt out of your aquarium is through your water changes – and you will only remove the proportion of salt that is dissolved in the amount of water you remove.

Only replace the same amount for every gallon you’ve changed.
If your fish aren’t responding well and you can see some issues, consider removing them from the tank and doing a full water change.
Are they listless? On their sides? Swimming erratically?
It’s better to act fast and be safe, than to wait and let your fish suffer.

There are five simple steps to performing a salt bath to get rid of parasites or fungus.

As this article has shown, there is absolutely no consensus on the question of whether or not you should use salt in your aquarium.
There are as many arguments for its use as there are against.

Those who are for it, say that it can support osmoregulation and prevent parasites and disease.

Those who are against it say that there are no long-term benefits to adding salt to your aquarium water, and that the potential risks outweigh any positive consequences.

What everyone DOES agree on, however, is that salt treatment can be a useful way to eradicate parasites and fungus when used carefully and for very short periods.
As a first-aid tool there are some benefits, however there are no clear guidelines or recommendations.

If you are considering using salt in your tank, it is critical that you do your research before adding anything to your aquarium water. Make sure you calculate doses correctly, and always be prepared to act fast if your fish start to show distress.