The uses and misuses of salt in freshwater aquaria rate their own page. In the fourth century B.C. Aristotle placed freshwater fish into seawater to observe their reactions. The aquarist's oldest treatment for freshwater skin and gill parasites is salt. "Salt is Nature's remedy for many ills," wrote William T. Innes in Exotic Aquarium Fishes, 1935, and salt as a medication survives from that era, when aquarists had very little else to turn to. A salt dip or a salt bath still has uses. Careful! an effective salt bath will kill many plants. Conversely, how effective on ciliate parasites is a salt bath that doesn't kill plants? Temperature should be raised to 80^oF. Mr. Innes' recommended concentration began with two level teaspoons of salt per gallon (the equivalent of 0.2% salinity) and built up gradually over 24 hours to four teaspoons/gallon. If no improvement was noted, by the third day you could go to six (that's two level tablespoons), as long as the fish were showing no signs of distress. "At the end of treatment, slowly add fresh water until the salt content is low before returning fish to the aquarium." Modern variants of the salt dose vary: a common version recommends just half of Mr. Innes' concentrations.

Salt tolerance in freshwater fishes varies. The percomorph fishes, like cichlids and anabantoids, are derived from marine ancestors in the age of dinosaurs. In general, they are more salt-tolerant than ostariophysii, the loaches and minnows, characins and catfish that have descended from freshwater ancestors. Years ago, William T. Innes reported that when a range of freshwater fishes were exposed to salt baths, the first to die were Corydoras. Those ostariophysan fishes that navigate by electric fields, like knife fishes, gymnotid eels and the "elephant noses" or mormyrids, should never be exposed to salt baths, according to the University of Florida. Killifishes and livebearers are more tolerant of salt, on the whole. But there are many exceptions to this very broad rule. Be aware of the salt tolerance of your particular fish.

Salinity measurements. A 3ppT or 5 ppT (parts per thousand) salt solution is equivalent to 0.3% or 0.5% salinity, or 3 or 5 grams per liter, or 3 or 5 teaspoons of salt per gallon. Remember, a level tablespoon is equivalent to three level teaspoons. This is the standard recommendation for salt treatment for skin parasites and for Ich.

Salt basics from U. of Florida. If you truly want to consider the effects of salt, it's good to know where to look. So start off by reading Dr Ruth Francis-Floyd's article "The uses of salt in aquaculture," the basic information presented by the U. of Florida's Institute of Food and Agriculture Science for the use of Florida's many fish farmers. The article outlines benefits of rock salt (a.k.a. kosher salt), both in controlling some parasites and in minimizing nitrite stress during shipping and handling:

Robert T. Ricketts' article, "The Salt of the earth... the salt of the sea," in Tom Griffin's AquaMag is essential reading. Exploding Urban Myths about toxic additives and salt as a tonic as he goes, RTR also gives good explanations of general hardness (GH) and alkalinity or "carbonate" hardness ("KH").

Some aquarists have always maintained a low level of regular table salt, NaCl, in the aquarium water, to "prevent" disease. Typical levels range from one teaspoon per ten gallons to as much as one tablespoon per five gallons. If you have any doubt about whether this is a magical practice or not, just try to dissuade someone from adding that pinch of salt, "just in case." From the reaction you get, you'll quickly recognize that you're in the land of magic.

RTR made some useful points about rock salt, compared to marine salts in an AC post, Nov. 2000:

"Brackish and estuarine fish are able to tolerate and likely to prefer more than only slightly higher Total Dissolved Salts, or osmolarity. In nature this is coupled with increased pH from the other salts dissolved in the water: calcium and magnesium carbonates and bicarbonates prominently. These materials "buffer" the water at higher pH levels than waters lacking these ions. So in the wild these fish arose in or frequented waters with ALL these ions and more. Adding salt, NaCl, alone will not affect the pH at all. Adding marine mix will provide a situation more like that from which the fish's forebears came. So long as the levels are below those that would register on a full range hygrometer, short-term exposure will do little harm (disease treatment), but long-term exposure should be reserved for fish with the physiology to live in that situation. I suspect that part of the reason many livebearers do well in this situation is that it is also more stable than an unbuffered tank, and for many fish, stability is as important as absolute pH, GH, or KH, provided it is within a ballpark of their "natural range". While some salt will certainly not peel off a cory's armor, it is not right for the fishes' physiology. Short-term is okay (provided you don't make sudden changes of too large a magnitude), long-term is likely to mean shorter life--— you can call it another unwarranted stress. And by the same token, water without the full mix can be a stress for those adapted by physiology over generations to having such conditions."

"I'm far from a purist about water," RTR continued,

"and rarely suggest folk go through the hassle of doing water modifications. I seldom do so myself. Most fish are quite adaptable. But if I do water mods, I want them to be in the right direction and for sound and understood reasons. I will not do so because popular myth says it is good for all fish in all water at all times. I know better than that, and wish more hobbyists would think about why they do the things they do, especially when they are adding things to the water."

Here's a useful warning from Nomad:

"I have treated fish for Ich using the salt/heat method, bringing salt concentrations up to ~1 tblsp/5 gal concentration. The trick when doing this with corys in the tank is to go slowly. Predissolve the salt in tankwater, and add it back into the tank a little at a time over several hours. When I've done this, I've never seen any signs that the fish in question were in any stress whatsoever. Just dumping in the salt, OTOH, can lead to them going belly-up pretty quickly. As with other parameters (pH, hardness, etc.) the real stress is not generally caused by the actual values in question, but by the speed of the change. Fish should be acclimated to higher salt levels the same way they're acclimated to differences in pH or hardness."

A note about salt and evaporation: You know that salt doesn't evaporate from aquarium water, of course. Though you may ignore my advice, and add a little salt to the aquarium, you know better than to re-salt the water after each partial water change, of course. So you are careful to add it only to the make-up water each time, at the recommended dosage. But! you need to top up the tank first, with unsalted water, to make up for evaporation, or else you are slowly replacing evaporation with salted water. In the course of a year, this will add up to a considerable dose of salt.

Salt dips and salt baths. Solutions of salt-- sometimes right up to marine concentrations (35 parts per thousand)-- are recommended as a short-term prophylactic bath for half an hour to remove parasites from gills, from fins and the outer surface of the epidermis, as Mr. Innes recommended so long ago. Encysted parasites, like mature Ich, are less likely to be affected. Intestinal parasites aren't affected at all. Chicago's Shedd Aquarium bathes their new freshwater arrivals in full-strength seawater to rid them of ectoparasites, without losses, said a curator there in a TFH interview a couple of years back. It's smart to pass your own new arrivals through a half-hour salt bath before putting them in the quarantine tank. A five-gallon covered bucket with an aerator is what you need; follow the link for details. The salt concentration has to be strong enough to make gill flukes drop away. Build up your dosage with pre-dissolved Kosher salt in two or three increments and stand ready to do a 50% water change with aquarium water at the first signs of stress; heavy respiration is an early signal. You'll want to do a 50% water change anyway, when the time's up, before netting them into the QTank.

Rock salt. Kosher salt. "Aquarium" salt. Marine salt. Rift Lake salts... What kind of salt to be using? "Salt" is the generic term for the dissolved ions and mineral component of all natural waters, which covers salts like Epsom salts, which is magnesium sulfate, characteristic of the spa waters at Epsom in England. (No, not "Epsom's" salt; "Dr. Epsom" will not be in today--— or ever!)

"Sea water is still better than salt crystals," Mr. Innes wrote in 1935. I'd say "different" rather than "better." Sodium chloride represents only about 77% of the total dissolved solids in seawater, and marine salt's dissolved carbonates will raise your pH. When we're talking about just plain "salt" we mean NaCl, sodium chloride, which is kosher salt or rock salt ("Animal feed grade" rock salt meets high standards of purity.) "Halite" is simply a lower grade of rock salt. Impurities may color it gray or even brownish. The "halite" sold for de-icing driveways, etc. isn't pure enough for aquarium uses, as you already surmised. "Table salt" often has iodine added, not the mythic poison it's made out to be, an issue separately addressed in a few moments. Table salt often contains as well some free-running additives, like magnesium carbonate, magnesium stereate or silicon dioxide--— all harmless. "Aquarium" salt is nothing but marine salt ("sea salt") that has been repackaged for the hobby and presented by colorful non-union toons in lab coats; check the label. Marine salts and Rift Lake salts contain many trace elements ("electrolytes") in addition to plain therapeutic NaCl. They are not substitutes for rock salt, nor vice versa. Nor substitutes for one another. Rift Lake waters actually contain surprisingly low concentrations of sodium chloride. As RTR pointed out, the other solutes in marine salt and Rift Lake salts will boost pH and hardness. You don't want to needlessly change those parameters as part of a medicating salt regimen.

Now I even hear of "freshwater salt!" Cheeky!

But don't completely ignore the salt packaged for the aquarium hobby: "Aquarium" salt also makes great pasta! And when guests drop over, set the "Aquarium" salt box on the table next to the pepper mill for a sparky conversation starter! They'll be impressed that you paid so much for salt!

Iodized salt. RavenSedai posted at Tom's Place, 23 July2000, the following from a koikeeper newsgroup, which pinpoints the anxiety about iodine in salt, an anxiety I don't share myself. Here it is, so you can decide whether to avoid iodized salt or not:

"Iodized salt has 77 ppm iodide for the prevention of goiter. Ocean water typically contains 0.006 ppm iodide, so sea salt is typically 0.2 ppm iodide. So if iodized salt were used in the pond, the iodide concentration would be roughly 400 times higher than if sea salt were used to produce the same salinity. In natural waters (ocean or freshwater), the predominant form of iodine is the oxidized form, iodate (IO3-), not the reduced form, iodide (I-). These two forms do not interconvert readily in water. The active ingredient in Betadine disinfectant is 1% iodide. The USDA suggests a 10 minute bath of 100 ppm iodide as a disinfectant for trout eggs. A permanent concentration in the pond of about 0.1 ppm, a weak disinfectant bath, sounds like an experiment, not like prudent advice. All of the USDA aquaculture publications specifically recommend non-iodized salt. The toxicity of iodide to fish is unknown. It seems likely that fry might be more affected than mature fish. In humans, toxicity begins at about 2 mg/day, only 13 times the US FDA recommended daily allowance. For that reason, salt is iodized at much lower concentrations in Europe than it is in the US. Finally, why suggest experimenting by using much more expensive iodized table salt? I bought an 80 pound bag of solar salt crystals at Lowe's for $2.87. It's much cheaper, and safer."

Fears about toxicity of the iodine represented in table salt are still often expressed in warnings not to use iodized salt in the aquarium. Potassium iodide (sometimes it's sodium iodide) in U.S. iodized table salt ranges from 20 to 40 parts per million. So what part per million of iodide does that potassium iodide represent?

I'm innumerative. The ppm iodide were worked out by aquariaddictus in a thread at AquariaCentral, started 1 March 2003 (you can find it at AC: search "iodized salt ppm iodide"). Aquariaddictus pointed out that there is no elemental iodine in KI, just as there is no free chlorine in table salt. Iodide is I-, while iodine is I₂, as chloride is Cl- while chlorine is Cl₂. The terms aren't interchangable.

Potassium is number 19 on the periodic table, Iodine is number 53. So pure KI is 73% iodide. Thus the iodide in the potassium iodide additive is between 14.6 - 29.2 ppm in the dry salt.

KI weighs 15.38 grams per teaspoon. So, 15.38 grams/one teaspoon per gallon = 15.38 grams per 3.7854 liters = 4 ppm as KI. If you add iodized salt at the rate of one teaspoon per gallon, you are adding iodide in the range of 0.083 - 0.166ppm.

After laying out the calculations, aquariaddictus remarked, "All in all, I have to believe it's a drop in the bucket. Does anyone use a tablespoon/gallon except in times of severe disease?"


A salt resource. The Salt Institute, a non-profit association of salt producers, maintains a websitewith lots of authoritative information about sodium chloride. .


Some genuine effects of salt in freshwater: osmoregulation. If salt has an effect on freshwater fish, how would that be produced? Well, salt dissociates in water to its two ions, sodium (Na+) and chloride (Cl-). Normally when you're talking about beneficial "effects" of salt in aquarium water, you're talking about the effects of the chloride ion.

A fundamental effect of salt is upon osmotic regulation. Osmoregulation, as it's called, is the interior balance of ions ("electrolytes"), the dissolved salts in blood and tissue that each living organism must maintain. "Osmoregulation" is the regulation of osmosis, that is to say, the selective diffusion of ions through a semi-permeable membrane. The gill surface is an example of a semi-permeable membrane. So is the coating of a bacterium. Osmoregulation is as important on the cellular level as it is to a multicellular organism.

Salt concentration in the water affects the cells' ability both to retain water and solutes or to expell water or solutes: a more dilute solution--— freshwater, for example--— tends to diffuse across a membrane into a more concentrated solution, such as fish blood. The tendency is to equalize concentrations of solutes. Individual cells-- and fish eggs and sperm are single cells--— are less tolerant of osmotic pressure than multicellular organisms, most of whose cells are constantly bathed in blood and lymph fluids, which protect the cells with an osmotic and pH-stable environment. We take advantage of this vulnerability of single cells when we consciously stress a unicellular parasite with salt. And we are sometimes disappointed by the low hatching rate of tetra eggs in hard water.

Salt and nitrite uptake. The chloride ion of salt has the desirable ability to inhibit the uptake of nitrite into fishes' blood.

During shipping, fishes' excreted ammonia can form nitrite, and the addition of enough salt to make a 1% solution has been found to cut shipping losses by as much as 90% in commercial practice. Nitrite is toxic, as you know. Fish that are stricken with nitrite poisoning get lethargic. With higher levels they may gasp as if they were suffocating and die with their gillcovers open wide. The nitrite ion has the damaging habit of occupying the place on a hemoglobin molecule where oxygen ought to be carried. The resulting molecule, called "methemoglobin" carries no oxygen. Under the influence of high NO₂ levels, the fish may suffer from "brown blood" syndrome or methemoglobinemia (yeah! "Me-THEME-o-Globe-anemia").

Though the main effect of nitrite is on the oxygen-carrying component of red blood cells, it's recently been shown to suppress chloride cells in the gill lamellae, which play a major role in maintaing a balance of salts. (Download the abstract of a 2002 paper read by O.T. Ferreira da Costa and M. N. Fernandes of the University of Sao Carlos, Brazil, "Chloride cell changes induced by nitrite exposure...")

How much salt should you be adding to counteract nitrite? It is the chloride ion of salt that is effective, not the sodium ion. In order to be effective, the chloride-to-nitrite ratio should be five to one. So if nitrite tests at 1 ppm, you should add enough salt (as a temporary measure) to give a chloride level of 5 ppm. This corresponds to about 8.5 ppm of NaCl (table salt); very little--— a fifteenth of a teaspoon or just a pinch-- in ten gallons. In fact, your water quite likely already carries this much salt, without any extra dosing at all; at any rate, your normal partial water changes will dilute out additional salt after the crisis has passed.

By the way, that useful chloride ion could perfectly well come from another source: you could use potassium chloride instead of sodium chloride, and plants would benefit from the potassium. Calcium chloride, CaCl₂, has two chloride ions; though not as cheap as rock salt, calcium chloride proved in trials more than twice as effective. Check this abstract of an article reporting nitrite trials with striped bass, Morone saxatilis, (a marine fish, however). (Check correspondence at Patrick Timlin's website).

Now, try telling this to a friend who adds a pinch of salt to "ease stress."

This use of salt's chloride ion does not assuage high levels of cortisol that are arguably more responsible for "shipping stress" than simple nitrite poisoning. At any rate, don't be misled by vague references to "stress." The chloride ion of rock salt simply acts as a specific block to the uptake of nitrite formed from ammonia in shipping water.

Salt and cleaning. Salt brine is a good cleaning and disinfecting agent for freshwater aquaria and accessories. To make brine, stir rock salt into warm water until some salt crystals remain at the bottom of the container and won't go into solution. "You've got brine." Articles soaked in brine should be allowed to air dry before being thoroughly rinsed.

Some magical powers of salt in freshwater: protection, healing and stress. Aside from these genuine uses, salt is often given magical powers connected with healing.

Salt and "protection." First, "Salt can aid in the production of the slime layer," you'll be told. And so could many water-born toxins or irritants, in fact. You're already aware that the fishes' normal slime covering is produced by specialized cells scattered through the epidermis. To increase their mucus production, these cells could be stimulated in two ways. One way would be through the action of a hormone. Hormones stimulate secretory cells of all kinds. But no one is suggesting that salt contains a hormone or is imagining that salt is some chemical precursor of mucus. In the other way, these slime-secreting cells could be stimulated by an irritant. After all, many irritants and toxins trigger hormones., and salt in the water merely acts as an irritant. If you've ever inadvertently half-poisoned a fish, as I have, you know that one reaction of the fish to any stressful irritant is to increase mucus production. It is true that the increased flow of mucus can help slough off incipient parasites. To this extent, you could justify saying that salt in the water "protects the fish from parasites." I find this to be stretching a point. Salting the water to increase the mucus layer is like putting a drop of lemon juice in your dry eye to make it water.

I've recently read that ammonia acts to thin and break down the slime layer of marine fish. Certainly we all know its action as a surfactant when we add a capful of household ammonia to the dishpan. If this is true in saltwater, NH₃ might have a similar effect on freshwater fish. But surely you'd act to reduce ammonia levels in the water, rather than to compensate for ammonia by adding salt.

Salt and healing. The second common magical power attributed to salt is that it "aids healing." In a dry atmosphere, a salt solution helps "draw" degraded cells and pus material from the abraded tissues of a wound, thus helping to clean it. A painful procedure from old shipboard days, when lacerations from a flogging were prevented from festering by applying dampened salt, still echoes in the phrase "to rub salt in someone's wounds." This bracing concept can't be extended to a water environment. When you inquire closely from an ol' "salter" how salting the water would aid healing, you'll soon understand that it's always based on an idea of "hyperosmolarity," that tendency of a more concentrated solution to "draw" a less concentrated solution that is at the heart of osmosis. You can't "draw" a fishes' wound; brine that was more concentrated than its own blood and tissues would quickly kill a freshwater fish.

A peripheral thought. Salinity levels that approach brackish water would repress Saprolegnia-type water molds, which can attack necrotic tissue. That's not the active consideration, though, when folks say some salt added to fresh water "aids" healing.

Salt and "stress." There is another wide-spread misconception, that some salt permanently in the water is "easing the stress" of osmotic pressure. This misconception is actively encouraged by packagers of "aquarium" salt. You'll hear this old tale repeated so often that, if it came to a vote, it might be voted "true." Most likely, this mis-application of "stress" comes from the idea of osmotic "pressure." Any pressure, such as social pressure, must result in some "stress" to the organism, such as social stress--— that's the thought, anyway--— and if osmotic pressure could be reduced, or even equalized, so that the surrounding water were at the same concentration as blood and tissues, then osmotic "stress" would be reduced. The fish would have to do less metabolic "work" to maintain osmolarity in its blood and tissue. I've even been told that the energy saved could then be applied to fortifying the immune system. These "logical" conclusions aren't based on the actual physiology of freshwater fishes.

But this is a mis-reading of the meaning of "pressure" in this case. Though peer pressure may result in stress, not all pressures result in stress. For example, atmospheric pressure doesn't result in stress. And neither does osmotic pressure. Fishes have evolved to adjust within certain limits to salt levels, ranging from virtually zero to water more saline than average seawater. No species of fish however will thrive at every level. Not all fishes are tolerant of change: the few that are warrant a special designation, "euryhaline." You know these things. But you will still hear lots of well-intentioned talk about salt and "stress," often from quite experienced aquarists. Listen skeptically, and I think you'll recognize this basic mis-connection between "pressure" and "stress."

Other opinions of magic powers of salt are just chit-chat. Take them with a grain of, um, whatever...

Or compare this good cautious article from the Aquascience Research Group, which may dissuade you from adding salt to the freshwater aquarium, if I haven't been able to convince you.

Salt sellers. Okay, now you're prepared to decipher claims made for an "aquarium" salt by its packager. Here, word-for-word, are salt claims off a box of "Aquarium" salt:

"An all natural* salt, made from evaporated sea water**...providing the essential electrolytes*** fish need to survive in an aquarium: calcium chloride, calcium sulfate, magnesium chloride, magnesium sulfate, potassium chloride and sodium chloride."

*"All natural." Generic reassurance.

**"Evaporated sea water" will contain many salts beside sodium chloride. Calcium and magnesium carbonates, not listed, would affect the pH buffering and would raise the pH in unbuffered acidic water. Additional buffering may make it more difficult to reduce pH if you're administering a medication that is only effective at a lower pH. The sulfates and chlorides listed provide no pH stability.

***"Electrolytes" are any dissolved salts. Of course you know that fish can't survive long in pure distilled water alone. It's in this sense that some minimal electrolytes are "essential."

"Helps improve gill function* to reduce stress**... Reduces electrolyte loss and promotes healthy gill function."

*"Gill function" in this case refers to osmoregulation, the regulated passage of ions back and forth across the gill membrane. Salt's chloride ion blocks the uptake of nitrite. It's inconceivable that "gill function" could be identified with the gills' certral role in respiration.

**"Reduce stress" refers to osmotic "stress"

"Can be used with most* aquarium remedies to improve recovery from disease."

*Though this claim reassures you that salt will not interfere with the effectiveness of medications, "most aquarium remedies" must exclude all those that are rendered less effective by higher pH.

"Protects fish against nitrite toxicity.*"

*Yes! Yes, at last! Indeed, minute concentrations of chloride ions do inhibit the uptake of nitrites through the gill membranes.