Aquarium salt: what it does, when to use it, safe amounts, and which fish can't take it

Aquarium salt is plain sodium chloride, the same chemical as table salt but without iodine or anti-caking additives. It is a useful tool for a few specific jobs, such as protecting fish from nitrite, treating some skin parasites and easing stress in a hospital tank. It is not a general tonic to add to every tank, and some fish, plants, shrimp and snails cope with it poorly.

Salt doesn't evaporate. When water evaporates, the salt stays behind, so you only add salt to replace what you take out in water changes. Getting this wrong is the most common way people overdose.

What you need to know first

What aquarium salt is, and isn't

Aquarium salt: sodium chloride (NaCl), usually as coarse crystals. Any pure, non-iodised salt with no anti-caking agents works the same way: pure kosher salt, pickling or canning salt, or rock salt sold for food or livestock. Read the ingredients: it should say only salt (sodium chloride).
Table salt: often contains iodine and anti-caking chemicals. Avoid it for fish.
Marine (reef) salt: a full mix of minerals for making sea water. It is not the same thing, and costs far more.
Water-softener salt: may contain additives. Avoid unless it is labelled pure.
Epsom salt: a completely different chemical, magnesium sulfate. It is used as a laxative or a short bath for bloating and constipation, not as a replacement for aquarium salt. See bloated fish.

What salt does

Blocks nitrite. The chloride part of salt competes with nitrite at the gills, so less nitrite gets into the blood. Fish farmers keep chloride at several times the nitrite level (extension guides suggest roughly 10 parts chloride to 1 part nitrite) to prevent "brown blood disease". See nitrite poisoning.
Eases osmotic stress. Freshwater fish constantly lose salts to the water. Injured or stressed fish lose them faster. A little salt narrows that gap, so the fish spends less energy balancing its body fluids. This is why salt is used in transport water and hospital tanks.
Kills or weakens some external parasites. Salt baths and long-term low doses are a classic treatment for some single-celled skin parasites, such as Epistylis and some cases of costia, trichodina and chilodonella. Many keepers also use it against ich, though results vary.
Increases slime coat production for a while, which can help protect skin.

What salt doesn't do

It does not cure bacterial infections such as columnaris or fin rot on its own, though it may support recovery.
It does not fix poor water quality, ammonia or overcrowding.
It is not "natural" or "needed" for most freshwater fish, and adding it all the time can stress fish and plants that come from soft water.

Step by step: using salt safely

Decide why you're using it. Nitrite spike, a known parasite, or a hospital tank for an injured fish are good reasons. "Just in case" isn't.
Check your fish, plants and invertebrates against the sensitive list below. If in doubt, treat in a separate hospital tank. See quarantine and hospital tanks.
Work out the real water volume. A "100-litre" tank with gravel and decor usually holds 80 to 90 litres.
Weigh the salt with a kitchen scale. Spoon measures vary a lot with grain size.
Dissolve it first in a jug of tank water, then pour it in slowly, near the filter outlet.
Raise the level in steps for anything above about 1 g/L, for example over 12 to 24 hours, while watching the fish.
Follow the label of your product, and stop and do a water change if fish show stress (rapid breathing, darting, lying on the bottom).
Keep track of salt in water changes. Add salt only for the water you replace, calculated for that volume. Top up evaporation with plain fresh water.
Remove it gradually when finished, with normal water changes without salt over a week or two.

Typical concentrations

These are ranges used in fish health and aquaculture references, stated in grams per litre (g/L). 1 g/L is 0.1%, or about 3.8 g per US gallon. Always follow the label of the product you're using and start at the low end, especially in a mixed community tank.

Use	Typical range	Notes
Low, long-term (hospital tank, stress, transport)	1 to 3 g/L	Aquaculture uses 1 to 2 g/L as an "indefinite" treatment and 1 to 5 g/L in transport water. Many hobby guides treat 1 g/L as a supportive dose.
Nitrite protection	Often well under 1 g/L	The aim is enough chloride to outnumber nitrite many times over. Small amounts go a long way. Fix the cause of the nitrite too.
Short bath (minutes to under an hour)	10 to 30 g/L	For hardy fish only, in a separate container, with the fish watched the whole time. Remove the fish at the first sign of distress.

The gap between a useful dose and a harmful one is narrow, and too little salt is also a common reason treatments fail. If you're unsure, ask a fish vet or an experienced keeper before you start.

Who doesn't like salt

Scaleless and fine-scaled fish: many catfish, loaches and some others are widely reported to be salt-sensitive. Hobby guidance often caps them at about 1 g/L for short courses.
Corydoras and other catfish, including plecos: commonly listed as sensitive. A small veterinary study found Corydoras aeneus mostly survived up to 2 g/L for 13 weeks, but the authors felt salt may still stress them. Low doses and short courses are the safe approach.
Soft-water fish: many tetras, rasboras, discus and wild-type fish from blackwater rivers.
Live plants: many aquarium plants show damage from around 1 g/L or less, and long-term salt weakens them. Move fish to a bare hospital tank instead.
Shrimp and snails: often more sensitive than fish. Don't salt a shrimp tank.
Tolerant fish: goldfish, many livebearers (mollies, guppies, platies) and some cichlids handle salt well. Mollies in particular often do better with a little salt, and some keepers keep them in brackish water.

Common mistakes

Adding salt to replace evaporation. This slowly concentrates it. Top up evaporation with fresh water only.
Adding a full dose at every water change. Only replace the salt you removed.
Using iodised table salt or salt with anti-caking agents.
Salting a planted or shrimp tank to treat one fish.
Mixing salt with other medicines without checking. Some combinations are harsher than either alone. Remove carbon from the filter only when a medication label says to.
Stopping too early. If you are treating a parasite, finish the course, then remove salt slowly.
Confusing Epsom salt with aquarium salt.

Products that help

Pure non-iodised salt (aquarium, kosher, pickling or food-grade rock salt without additives): the actual treatment.
Kitchen scale: weighing is far more accurate than spoons.
Hospital tank or large tub with heater and air stone: lets you salt one fish without harming plants or invertebrates.
Salinity or chloride test, or a TDS meter: helps confirm the level if you treat often.
Nitrite test kit: for deciding whether salt is needed for nitrite protection.
Epsom salt (magnesium sulfate): a separate tool, for bloating or constipation baths only.

Why salt has such a mixed reputation

Salt is cheap, widely available and genuinely useful, so it became a cure-all in the hobby. At the same time, many sensitivity warnings come from experience rather than controlled studies, and some are probably overstated. The safest view sits in the middle: use salt for a clear reason, at a measured dose, for a set time.

When to get help

Ask a fish vet or experienced keeper before using salt on a tank with catfish, loaches, shrimp, snails or delicate soft-water fish, or before using a short high-strength bath for the first time. If fish are gasping, test for ammonia and nitrite first: salt is not a substitute for fixing the water.

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