Aquarium salt: what it does, what it harms, and what it cannot treat

Quick answer

Salt is the most reached-for product in freshwater fishkeeping and the least understood. It does three specific things and nothing else.

Aquarium salt is sodium chloride. It genuinely helps in three situations: it protects fish against nitrite poisoning, it reduces the osmotic work a damaged fish has to do, and it kills the free-swimming stages of some external parasites. It does none of the other things it is recommended for.

Two facts decide whether it helps or harms. Salt does not evaporate, so topping a tank up without accounting for it makes the concentration climb indefinitely. And several popular fish, including most small catfish, loaches and many soft-water species, tolerate it badly.

• Salt is not a medicine. It does not treat internal bacterial infection, internal worms, fungus reliably, or the causes behind dropsy and swim bladder problems.
• Salt does not leave the tank when water evaporates. Only a water change removes it.
• Dose by the actual water volume, not the number printed on the tank. Substrate and decor can displace a large fraction of it.
• Scaleless catfish, loaches, many soft-water tetras and rasboras, live plants, shrimp and snails all tolerate salt poorly.
• Always pre-dissolve salt in a jug of tank water. Never sprinkle dry crystals into an aquarium.

Decide in 60 seconds

What salt actually does

It protects against nitrite.

Nitrite is taken into a fish through the same gill pathway that handles chloride. Once inside, it converts the blood's haemoglobin into a form that cannot carry oxygen, and the fish suffocates in fully oxygenated water. The blood turns brown, which is where the old name brown blood disease comes from.

Raising the chloride level in the water means chloride competes for that uptake pathway and less nitrite gets in. This is the single best-supported use of salt in freshwater keeping.

It is support, not treatment. Nitrite in a home aquarium means the biological filter is not keeping up, and the actual response is a water change and finding out why the cycle is failing.

It reduces osmotic work.

A freshwater fish sits in water far more dilute than its own body fluids. Water floods in across the gills and skin, and salts leak out, continuously. The fish spends energy pumping water out through the kidney and pulling ions back in at the gills.

Damage anything that separates the fish from the water, a torn fin, an ulcer, ammonia-burned gills, a parasite-scarred flank, and that leak worsens. Raising the salt level in the water narrows the gradient and reduces the work.

That freed energy goes into healing. Salt does not close the wound or kill the bacteria. It removes a tax the fish was paying while trying to recover.

It kills some external parasites, at one stage of their life.

Several external protozoa and flatworms have a free-swimming stage in the water column. Those stages are small, thin-skinned and osmotically fragile, and raised salinity kills them.

The stages that are embedded under the fish's skin or encysted in the substrate are protected. This is exactly why salt regimes appear to work for a few days and then fail: the free-swimming stage was killed, the sheltered stage was not, and treatment stopped before the next generation emerged.

What salt does not do

Salt is not an antibiotic. It has no useful action against internal bacterial infection, and it does not treat the septicaemia that lies behind many of the presentations owners reach for it.

It does not treat internal parasites and worms, which are handled through medicated food or specific treatments.

It is not a reliable antifungal. Some true fungal infections respond to other treatments, and several conditions that look like fungus to the eye are bacterial and progress fast.

It does not treat the swollen, pineconed fish. That appearance is fluid accumulation from organ failure or systemic infection, and it needs a diagnosis rather than a salt dose.

It does not fix buoyancy problems, which have causes ranging from feeding and diet to body shape, infection and physical compression of the swim bladder.

It does not add oxygen. A gasping fish in a warm tank needs surface agitation and cooler water, not salt.

Perhaps most importantly, it does not tell you what is wrong. Salt is offered for every symptom in every forum thread, and every hour spent dosing it is an hour the actual cause continues.

The three salts people confuse

Aquarium salt, tonic salt, or plain non-iodised sodium chloride.

The product this article is about. Chemically it is sodium chloride, and where a purpose-made aquarium product is unavailable, plain salt with no anti-caking agents, no iodine and no additives is the same compound. The concern with table salt is the additives, not the sodium chloride.

Marine salt mix.

Synthetic sea salt, containing many ions and carbonate buffers as well as sodium chloride. It raises pH, hardness and alkalinity substantially. Using it where aquarium salt is meant changes the whole water chemistry of a soft-water tank.

It is the correct product for a marine tank, and for genuinely brackish species such as mollies kept hard, monos, scats and some puffers and gobies. For those fish this is not medication, it is their habitat.

Epsom salt.

Magnesium sulphate. A completely different chemical with a different use, mainly as an osmotic agent for constipation and swelling. It is not a substitute for aquarium salt and it does not do what aquarium salt does.

Reaching for the wrong one of these three is one of the commonest errors in this whole subject, and the packaging in many shops does not make the distinction obvious.

The two mistakes that cause the harm

Salt does not evaporate.

Water leaves an aquarium as vapour. The salt stays behind. That means the concentration rises every time water evaporates and is topped up with fresh water containing no salt, which is the correct thing to do, and rises further if the top-up water is salted, which many keepers do believing they are maintaining a level.

Only a water change removes salt. Left alone, a tank that has been salted repeatedly and topped up rather than changed drifts upward toward a level nobody has measured and nobody intended.

This is why "keep a bit of salt in the tank permanently" is bad advice for anything other than a genuinely brackish setup. There is no such thing as a stable maintenance level maintained by topping up.

You do not know your tank volume.

The figure printed on a tank is the volume of the empty box, filled to the brim. Your tank contains substrate, rock, wood, ornaments, equipment and a gap of several centimetres under the rim.

Between those, the actual water volume can be substantially less than the printed figure. Dosing anything, including salt, by the printed number overdoses the tank, and the error is largest in small tanks with deep substrate and heavy hardscape, which is exactly where fish have the least margin.

Measure it once. Fill the tank from a container of known volume during setup and write the total on the hood, or calculate from the internal water dimensions and then subtract for substrate and decor. Every dose you ever make afterwards depends on that number.

The smaller the water volume, the larger every dosing error becomes, and the faster salt concentrates as water evaporates.

The fish and plants that cannot take it

Salt tolerance varies enormously and it is not obvious from looking at a fish.

Scaleless and thinly scaled fish.

Small catfish including Corydoras, and the suckermouth catfish, exchange far more across their skin than a heavily scaled fish does. Loaches, including kuhli and clown loaches, are in the same group. These fish show distress at levels that a livebearer would not notice.

Soft-water and blackwater species.

Many tetras, rasboras, dwarf cichlids and wild-caught soft-water fish come from water that is low in dissolved ions. Raising the ionic content is moving them further from their physiology, not closer.

Invertebrates.

Freshwater shrimp and snails are generally less tolerant than the fish they live with. A salt dose calculated as safe for the fish can be fatal to the shrimp colony.

Live plants.

Many aquarium plants are intolerant, and salt is a common and rarely diagnosed reason a planted tank melts a week after a treatment.

The practical conclusion is that whole-tank salting is rarely the right approach in a mixed community aquarium. If salt is genuinely indicated for one fish, the fish moves to a separate container, not the salt into the display tank.

How to use it correctly, if you are going to

Test the water first.

Ammonia, nitrite, nitrate, pH and temperature. This decides whether salt is relevant at all, and it gives the vet or aquatic specialist something to work with if it escalates.

Know your real volume.

As above. Guessing here undoes everything else.

Pre-dissolve.

Dissolve the salt fully in a jug of tank water, then pour the solution in slowly near the outflow so it disperses. Never sprinkle dry crystals into the tank. A fish will investigate and eat one, and a shrimp or snail sitting on an undissolved crystal is in contact with an extreme local concentration.

Add it during or after a water change.

That way you know the starting point. Adding salt to water that already contains an unknown amount from a previous dose is how tanks drift.

Follow the product's stated rate for the stated purpose, and do not add extra.

Rates differ between products and between purposes, which is why this article gives none. What is universal is that more is not better and that a doubled dose in a mis-measured tank is a large overdose.

Watch the fish.

Rapid gilling, loss of balance, rolling, sudden hiding, or a rush to the surface after dosing means the fish is not tolerating it. Do a water change immediately.

Decide how it comes back out.

Plan the water changes that will remove it before you put it in. Activated carbon removes many medications but does not remove salt. Nothing removes salt except replacing the water.

A water change is both how salt is introduced sensibly and the only way it leaves. Plan the second before you do the first.

Baths versus whole-tank dosing

A bath is a short exposure at a higher concentration, in a separate container of tank water, with the fish watched continuously. It has the advantage of not touching the display tank, its plants or its invertebrates.

It also has to be supervised without exception. A fish that rolls, loses its balance, gasps at the surface, or lies on its side goes straight back into the main tank. There is no "give it another minute".

Whole-tank dosing is a long exposure at a low concentration. It has the advantage of reaching parasites in the water column, which is often the point, and the disadvantage of exposing every other living thing in the tank for as long as the salt is present.

For most community aquariums, if salt is indicated, a hospital container is the safer route. Reserve whole-tank dosing for situations where the parasite life cycle in the tank is the actual target and the stocking will tolerate it.

The interaction people miss

Salt is frequently used alongside raising the temperature, because warmth speeds parasite life cycles through the vulnerable stage.

Warm water holds less dissolved oxygen. A fish with parasite-damaged gills, in warmer water, in a tank with limited surface movement, can suffocate while the treatment is technically working. Increase aeration and surface agitation whenever you raise the temperature, and do it before rather than after.

Salt also interacts with some medications, and combining treatments without knowing how they interact is a common way to lose the fish you were trying to save. Change one thing at a time and record what you changed and when.

Raising temperature alongside salt is common and reduces dissolved oxygen at the same time. Add aeration before you add heat.

The routine that makes salt unnecessary most of the time

Almost every situation in which a keeper reaches for salt is one that regular testing and quarantine would have prevented or identified.

What never to do

Do not keep salt in a freshwater tank permanently as a preventive. There is no stable maintenance level, the concentration drifts upward with evaporation, and the plants and invertebrates pay for it.

Do not dose salt as a reflex before testing the water. This is the single most common error in the subject.

Do not sprinkle dry salt into a tank.

Do not use marine salt mix where aquarium salt is meant, unless you intend to change the tank's hardness and pH.

Do not salt a tank containing loaches, small catfish, shrimp, snails or live plants without a specific reason and an understanding of what you are risking.

Do not combine salt with medications, temperature changes and water changes all at once. When something then goes wrong, you cannot tell which change did it.

Do not assume a fish that stops showing symptoms is cured. With parasites in particular, the visible stage disappearing is a stage of the life cycle, not the end of the problem.

When to get help

Contact an aquatic or exotics veterinarian, or an experienced aquatic specialist, for any of these:

• Several fish showing signs at the same time, which almost always points at the water or at something recently added
• Any fish with an open ulcer, a rapidly spreading white or grey growth, or bleeding at the fin bases
• A fish that continues to deteriorate after the water has been tested and corrected
• Suspected parasite outbreak in a tank containing species that cannot tolerate the usual treatments
• Any fish that has already been dosed with several products in combination

Bring these to the consultation: your water test results with dates, the tank's measured water volume, how long it has been running, a complete stocking list including plants and invertebrates, every product added in the last month with the rate and date, whether activated carbon is in the filter, and photographs or video of the affected fish.

A dosing history is often more useful than an examination. A large proportion of the cases that reach an aquatic vet are not a disease at all but the accumulated result of several well-intentioned treatments added on top of each other.