Frequently asked questions on using salt


Salt has been scientifically proven to have a number of benefits. Matt Clarke scours the journals and speaks to industry experts for the latest on the subject.

Many people are very anti-salt. Why is this, and why on earth would you want to add salt to a freshwater tank, anyway?

Unfortunately, there are lots of myths and old wives' tales surrounding the use of salt. While it is sometimes used inappropriately, it does have a number of genuine, scientifically valid uses in the aquarium for treating health problems, tackling water quality problems and minimising stress, especially during transport. Most of these work by salt's effect on the osmoregulation of freshwater fish.

What is osmoregulation? Is it to do with osmosis?

Osmoregulation is the technical term for the physiological mechanism fish use to control the amount of salt and water in their bodily fluids. As the name suggests, it's based on osmosis.

If you are young enough to remember your schooldays, you'll recall that this is the movement of dissolved stuff through a semi-permeable membrane from a strong concentration to a weaker one.

Basically, freshwater fish are saltier than the water they live in, and their skin is semi-permeable. Since there's a big difference between the amount of salt on the inside of the fish and the amount of salt in the freshwater they live in, freshwater fish leak bodily salts and take in water. Of course, this presents two problems for a freshwater fish: taking in salt and getting rid of water.

Osmoregulation tends to not work so well when the fish are stressed or diseased. This "osmoregulatory dysfunction" means that fish find it difficult to get sufficient salts from the water and may have problems getting rid of excess water.

How do freshwater fish get rid of excess water and take in salts?

Getting rid of the water that floods in is fairly easy. You just urinate constantly! Freshwater fish produce vast amounts of urine - some produce their own body weight in wee every three or four days.

If you were, for argument's sake, a 76kg/12 stone Guppy, you'd be able to fill a Juwel Rio 180 with your own urine in about a week!

Taking in extra salts to replenish those lost to the water takes a bit more effort. Freshwater fish use special cells in their gills to take in the salts, such as chloride, that they lose to the water, which helps keep their bodies salty. Adding salt to the water when fish are stressed means that they don't lose as much salt from their bodies. The principle is much the same as a saline drip for hospital patients.

Should salt be added as a routine tonic for freshwater fish?

Some shops and salt manufacturers recommend adding salt as a routine permanent treatment. However, while salt has some advantages, there is no need to dose with salt on a permanent basis. Most experts only advocate this for brackish species.

Leading fish vet Chris Walster says: "Freshwater fish should be kept in freshwater, not any other.

"We have no idea of what effect placing freshwater fish in salted water has from a welfare perspective. We know that if we place freshwater fish in seawater that they will die sooner or later. We simply do not know, even at a low level of salt, whether it irritates the eyes or gills, etc.

"When we swim in the sea, the salt irritates our eyes. Is it the same for freshwater fish? Are there any other unknown effects which occur weeks, months or years later?"

Fish health expert Dr Peter Burgess says he certainly doesn't advocate salt for permanent use: "Unless the species has a natural requirement for salt, then we should not add salt to an aquarium (or pond).

"Tonic salt for freshwater fish is a bit like aspirin for humans: both medicines have many beneficial uses, but neither should be administered routinely just for the sake of it. Bear in mind that most tropical community tanks will contain salt-sensitive species, such as catfishes.

"Salt can be used as a supportive for salt-tolerant species, for example if the fish have severe ulcers or other major skin breaches that can place a burden on the osmoregulatory system. But healthy, unstressed fish do not need this support. Never use salt to compensate for bad fishkeeping!"

Why, then, is salt added to freshwater systems?

When freshwater fish are stressed, they suffer from osmoregulatory dysfunction, which basically means that they lose bodily salts to the surrounding water. Stress causes this salt-water balance system to stop working properly and the fish suffer.

Adding salt to the water in times of stress can help minimise salt losses by the fish. It can also reduce the toxicity of pollutants and may kill some pathogens.

What problems can low doses cause?

Dr Peter Burgess told us: "I have heard that salt can damage pond plants and, of course, it can only be removed by the diluting effects of water changes. Also, the prolonged use of salt can make some pond fish parasites (eg Trichodina) more salt-resistant, which means even higher salt levels will be needed to eradicate these organisms."

Can salt really help to fight diseases, such as parasites?

Yes. Salt has been used for decades as a cheap and effective treatment against many diseases, particularly protozoan infections on pond fish such as Chilodonella, Trichodina and Costia, but also some other pathogens, such as gill and skin flukes.

It rarely has an adverse effect on filtration, unlike many other medications, and can either be used as a longer-term low-dose bath or a higher-dose short-term dip.

However, particular care needs to be taken when using strong salt baths, particularly if the fish are already weakened by disease.

Does this work the other way round? Can lowering the salinity in a marine tank eradicate parasites?

Yes, this may work for some marine parasites, but not for all of them, including marine whitespot, contrary to popular belief among those in the marine community!

Dr Peter Burgess, who holds a doctorate on this very subject, told us: "My friend Dr Angelo Colorni in Israel (another Cryptocaryon fanatic, like me!) found that freshwater dips, even long ones, for up to 18 hours, did not prevent development of the parasites in the fish's skin. I guess the skin partly protects the parasites from hypo-salinity effects.

"Colorni found that the free-living infective (theront) stage of Cryptocaryon was hypo-saline sensitive, being killed at salinities below 25 parts per thousand. Of course, most other common skin parasites live on the skin surface, not beneath it, as in ich/crypto, so are more vulnerable to low salinities."

Can salt clear the gills of mucus which builds up when fish suffer from parasites or poisoning?

Yes. According to fish vet Chris Walster, at lower dosages salt can clear the gills of excess mucus.

However, he suspects it may act as an irritant at higher doses: "It could promote mucus production, causing the excess to slough off. This effect probably would be short-term (stress to fish, depletion of mucus cells), and the benefits need to be considered against the disadvantages.

"As such I might use it in an emergency as a one-off treatment where the fish is suffering respiratory distress and no other alternatives were available."

Tetra's Rupert Bridges spoke to their research and development scientists.

He told us: "They suggest that adding salt increases the production of mucus, and therefore speeds the renewal of the mucus layer. The effect is therefore due to a higher turnover of mucus, which helps rid the gills of parasites, etc.

"In my opinion, this is a more satisfactory explanation than any reduction in toxicity of nitrite and ammonia, as salt generally works to rid the blood of excess ammonia and prevent nitrite take-up, not reduce its concentration in the water."

Are fry and young fish more sensitive to salt than adults?

Yes, lab studies have shown that fry are not as tolerant to salt as larger fish. Rothen et al. (2002) studied the effects of salt on the fry of Blue gouramis, Zebra danios, Black widow tetras and Buenos Aires tetras.

All fry tolerated up to 1ppt, and doses as high as 3ppt had little effect on Blue gourami fry. The tetras and danios were OK at 1ppt, but started to suffer at higher dosages. Exercise caution.

What exactly is "tonic salt"? Surely salt is just salt, isn't it?

Tonic, or aquarium salt, is basically plain old sodium chloride, often with a bit of anti-caking agent thrown in to make it easier to pour. Marine salt, on the other hand, is much more complex and contains special blends of minerals, like magnesium and calcium, which are useful for making the water better for marine invertebrates.

Tonic salt doesn't do this. It's designed for therapeutic use, not for making the water resemble that of the sea.

I've been warned off aquarium salts that contain sodium hexocyanoferrate. Why is this?

The additive sodium hexocyanoferrate is an anti-caking agent and is designed to make salt free-flowing. Without an anti-caking agent, salt tends to take up moisture from the air and form hard lumps which are harder to dissolve.

Most fishkeeping writers, myself included, have advised against the use of salt containing this - but I'd bet money on none of them knowing the reason why. I certainly didn't know why this stuff was said to be potentially toxic.

Sodium hexocyanoferrate contains cyanide! Run-off from roadside salt storage facilities, which contain salt containing sodium hexocyanoferrate used for salting icy roads, have been found by Ohno (1990) to contain high levels of cyanide!

Cyanide is toxic to fish at high levels, which is why its use to catch marine fish is frowned upon.

I've been unable to find any evidence in the journals which show that this is harmful to fish in the low levels that are likely to occur in the aquarium. Most suppliers of bulk tonic salt label their product as containing it. The salts are used extensively, and I've never heard of problems relating to their use.

So what's Malawi salt? I thought Malawi was a freshwater lake.

The "salt" recommended for use in Rift Lake cichlid aquaria is probably best called a mineral supplement, since it isn't based on sodium chloride. It's designed to make the water harder and more alkaline, not saltier.

Some people advocate adding tonic salt for Malawis, others reckon it can lead to Malawi Bloat. It is neither an essential nor a recommended addition to a Malawi tank, and we would never advise you to add it.

Malawi Salt, on the other hand, is handy if you live in a softwater area, since it effectively replenishes the missing minerals in your water.

What salt should I use in a brackish aquarium?

Brackish fish expert Dr Neale Monks says: "Marine salt mix. It's so inexpensive in the quantities brackish or freshwater require that there's no point economising with table or rock salt."

How do I calculate the correct dose to add?

Salt is normally dosed in grams per litre or parts per thousand. Both equate to the same amount: 1ppt is 1g per litre. This makes it easy to calculate how much is required if you know the aquarium or pond volume.

Which species are intolerant of salt?

This is a tricky question, because it's hard to confirm that problems seen in fish have been caused by administering salt. I've seen salt used widely on a range of supposedly salt-intolerant species with no obvious ill-effects.

However, many claim that catfishes, especially Corydoras, and Malawi cichlids tolerate salt badly - I've yet to see this myself, though. Research shows that many fish from soft, acidic water will tolerate salt.

Vet Chris Walster told us he had not yet seen a species that appeared intolerant. He said: "Where I have come across an unfamiliar species, I would always recommend a test treatment first. This test should be observed and the fish put into freshwater at the first signs of distress. Salt dosage is varied not only by strength but also by time. Having said that, I am aware that there is variation between species and within species on their tolerance to salt."

Tetra's Rupert Bridges says: "I have come across very few studies that have shown intolerance to salt (NaCl) levels as low as 1-3ppt. One study (Rothen et al. 2002) showed that danio (D. rerio), Gymnocorymbus ternetzi, and Hemigrammus caudovittatus fry displayed higher mortality rates at salinities of 1-3ppt, but that Blue gourami fry were tolerant.

"Often softwater fish are credited with poor salinity tolerance. I have also heard that catfish are cited with poor salinity tolerance, but some species do tolerate reasonably high levels. The general rule would be that salinities up to 3ppt are fine for coldwater fish, but that salt should only be used with caution (perhaps at 0.5-1ppt) or not at all for tropical aquaria."

Dr Peter Burgess also reckons that many catfishes and some other groups can be intolerant: "On an individual basis, freshwater fish that are weakened tend to be less salt-tolerant than healthy individuals."

How does salt affect the toxicity of ammonia to fish?

Ammonia is present in two forms in water: ammonium and free ammonia. Ammonium is not very toxic to fish, but free ammonia is. The ratio of dangerous free ammonia to less toxic ammonium depends on the pH, temperature and salinity. As salinity drops, free ammonia increases.

Importantly, ammonia test kits do not measure the level of free ammonia. Instead, they measure something called total ammonia nitrogen, a combination of free ammonia and ammonium. It is up to you to calculate how much free ammonia is present. The PFK website has a calculator to work this out for you.

Can adding salt reduce the toxicity of nitrite to fish?

Yes, salt reduces the toxicity of nitrite to freshwater fish. Salt is a compound containing sodium and chloride (NaCl). Studies have shown that chloride reduces the toxicity of nitrite to fish, so salt can be added to provide the chloride ions that offer the fish some protection against the nitrite.

Very importantly, the level of chloride required to do this need not be very high at all. Research suggests a dose of between 7:1 and 10:1 chloride to nitrite is all that is needed. So adding salt to protect against nitrite pollution should be safe even for salt-intolerant species, because only a tiny quantity is required. There is a Nitrite Toxicity Calculator on our website to calculate what you need.

Dr Peter Burgess agrees that too many people incorrectly assume that a high level of salt is needed to tackle a nitrite crisis.

Peter told us: "For counteracting the effects of high nitrite, just 100 mg (0.1g) salt per litre is enough. This very low salt level is tolerated by virtually all freshwater fish, even catfishes."

As we went to press, a new paper was published in the journal Environmental Science and Technology which looked at the physiological reason why some fish were more susceptible to nitrite poisoning. Again, it's all down to chloride.

Does salt affect nitrate toxicity?

Yes, there may be some evidence for this. Westin (1974) has shown that nitrate is more toxic in brackish water than in freshwater. New research on nitrate toxicity in freshwater and marine conditions has been published in Chemosphere.

This suggests that the main toxic effect of nitrate is due to its ability to convert oxygen-carrying pigments to forms that are incapable of carrying oxygen. It says that nitrate is technically less toxic in saltier water.

What is the normal therapeutic dose rate?

If fish have parasites, open wounds or osmoregulatory problems that cause dropsy or popeye, a dose between 1-5ppt is the norm, according to fish veterinary texts.

The salt is dissolved in a bucket of water before being added, and then left in the tank as a permanent bath. If you want to sustain the salt level over an extended period, add the same dose of salt to water changes.

Temporary 4-5 minute baths are only used in extreme circumstances. These use a high dose of 30-35ppt, which is about the same as full-strength seawater. Remove the fish if they appear to be getting stressed.

Fish vet Chris Walster says that the dosage required is very variable and should be looked at on a case by case basis: "Salt should not be used as a permanent bath at any dose. Its main use is as a first aid treatment until the correct diagnosis is found, or at low doses in support of other therapies."

He says a general dose of 1.5-3g/l. can be used as a long-term bath for up to two weeks, but that this can be doubled depending on the species and the size of the fish.

He says this is useful after transportation and as a supportive to other therapies, such as when fish are being treated for ulcerations. Smaller and younger fish should be started on doses of 0.5 ppt.

Says Chris: "As a short-term bath, anything up to 37.4ppt per litre with a time of 30 seconds to 10 minutes. This is useful for clearing excess mucus, and by secondary intent, removal and reduction of any pathogens on the surface of the fish."

This is an item from the Practical Fishkeeping website's archives. It may not be reproduced without written permission.

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