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Many fish stores, and other
“sources of wisdom” about fishkeeping, will recommend salt as a general
"tonic" for freshwater tropical fish. The usual
suggested dosage of salt is something like a teaspoon per 5 gallons.
As David A. Lass points out, there is not much therapeutic
benefit at those dosages. “Salt serves more to
assuage the hobbyist's need to ‘do something’ for their tropical fish,”
he writes.  There is absolutely no need to add
salt to freshwater aquaria except as a specific treatment, and even here
the sensitivity of certain fish species must be kept in mind.
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
As the scientific data presented in this summary article indicates, adding salt to a freshwater aquarium on a regular basis will, at best, do nothing of any value at all. But at worst, it will stress salt-intolerant fish, making them more vulnerable to disease and less likely to live a healthy and normal lifespan. To understand why, we need to understand what salt does in water, and how fish are affected. But before this, we must clarify just what we mean by “salt.”
In chemistry, salts are ionic
compounds that result from the neutralization reaction of an acid and a
base. They are composed of cations (positively
charged ions) and anions (negative ions) so that the product is
electrically neutral (without a net charge) [Wikipedia, definition of
“Salt (chemistry)”]. There are mineral salts for
most minerals. But for the purpose of this article,
we are dealing solely with common salt—what we know as table
salt, or rock salt, or aquarium salt. This salt is a
mineral that is composed primarily of sodium chloride (NaCl), a chemical
compound belonging to the larger class of ionic salts.
It is essential for animal life in small quantities, but it is
harmful to animals and plants in excess. Marine salt
has other minerals in it too, but it is still “salt” for the purpose of
Salt is an irritant, which
causes the fish to secrete more mucus particularly in the gills where
osmoregulation is occurring. And if salt is not
predissolved carefully, it can give fish bad burns; this is especially
true for scaleless fish, such as loaches, many catfish and some types of
Salt makes the water denser than
the same water without salt. The aquarium contains
water. The bodies of fish and plant leaves also
contain water, just as we do—humans are approximately 70% water.
The water in the aquarium and the water in the fish/plant are
separated by a semi-permeable layer which is the cell.
Water can and continually does pass through this cell; fish do
not “drink” because they don’t have to in order to take in water.
When either body of water is denser, the other less-dense body of
water will pass through the membrane to equalize the water on both
sides. The fish must control this process through
what is termed osmoregulation.
Freshwater Fish Physiology
Salt definitely interferes with
the osmotic regulation of fish and plants. It should
be left alone; nature regulated that part itself, by creating
freshwater, brackish and saltwater fish. The vast
majority of freshwater fish live in waters having no measurable
salinity, and this has been crucial in the evolution of their
physiology. Fresh water fish differ physiologically
from salt water fish in several respects: their gills must be able to
diffuse dissolved gasses while keeping the salts in the body fluids
inside; their scales reduce water diffusion through the skin; and they
also have well developed kidneys to reclaim salts from body fluids
Freshwater fish have
physiological mechanisms that permit them to concentrate salts within
their bodies in a salt-deficient environment; marine fish, on the other
hand, excrete excess salts in a hypertonic environment.
Fish that live in both environments retain both mechanisms.
Freshwater fish concentrate salts to compensate for their low
salinity environment. They produce very dilute but
copious urine—up to a third of their body weight each day—to rid
themselves of excess water, while conducting active uptake of ions at
the gills. 
The kidneys of freshwater fish
have two functions: osmoregulation [discussed below] and hematopoiesis,
which is the formation of blood celular components.
Each fish species is adapted to the range of salts in its habitat water,
and the kidneys function well within that range. The
kidneys have to work harder whenever the salt content of the water in
which the fish is living is greater than that of the fish’s preference,
i.e., the natural habitat. The closer the water is to
the species’ requirements, the easier it will be for the fish to
maintain proper osmotic levels. One of the myths
about the “benefit” of regular addition of salt is that it allegedly
maintains an osmoregulatory balance; in point of fact, regular use of
salt has the exact opposite effect and can cause bloating
due to an osmotic imbalance. 
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. Water
is constantly passing through the cells of freshwater fish by osmosis in
an attempt to equate the water inside the fish with the water in the
aquarium. Freshwater fish regularly excrete this
water through respiration and urination; the average fish will urinate
30% of its body mass every day. The more salt in the
aquarium water, the greater the strain on the fish's kidneys, which in
turn adds to the fish's stress in attempting to maintain their internal
And salinity affects the amount
of energy the fish must spend to maintain the physiological
equilibrium—the complex chain of internal chemical reactions that keep
the pH of the fish’s blood steady, its tissues fed, and the immune
system functioning. When salinity increases beyond
what the fish is designed by nature to handle, the fish must work harder
and use more energy just to “keep going.” Laura Muha
 likens this to driving a car up a steep hill—it takes more energy
(gas) to maintain the same speed as driving on level ground, and it
causes more “wear and tear.” This increased energy
output is wearing down the fish, and the fish is not able to expend this
crucial energy on other important functions. The
growth rate is affected, a shorter lifespan will usually result, and
there will be increased risk of various health problems along the way.
Fish and plants from
mineral-poor waters do not appreciate being kept in slightly saline
water conditions. Many of the most popular fish
today, like cardinal tetra and rasbora, come from soft water habitats.
Short term exposure to low salt concentrations across a few days
or a couple of weeks may not do them major harm, but constant use of
salt in their aquaria could cause problems.  In
Weitzman et al. (1996), the authors mention that 100 ppm of salt is the
maximum that can be tolerated by most characins, and some species show
considerable stress leading to death at a level of 60 ppm. 
To put this in perspective, 100 ppm is approximately equal to
0.38 gram of salt per gallon of water. One level
teaspoon holds approximately six grams of salt, so just 1 teaspoon of
salt in 16 gallons of water will cause stress, and in some species lead
Another problem is that salt
increases the total dissolved solids [TDS] in the water.
An aquarium treated with one teaspoon of salt per gallon of water
will have an established dose of 2400 ppm. Add to
this the TDS occurring from calcium and magnesium salts [these make
water “hard”], water conditioners and other additives, and you can end
up with over 3000 ppm of TDS.  This is intolerable for most fish;
even the very hard water in the African rift lakes does not contain more
than 600 ppm TDS. And for fish from naturally soft
and acidic water environments, this is very dangerous, for nowhere in
nature does acidic water exist with a level of TDS anywhere near this.
And the deviation from normal osmotic pressure that this creates
is very harmful to all fish.
Keeping the tank salty all the
time will not help with disease resistance in freshwater fish; in fact,
it will actually increase the fishes’ susceptibility to disease and
parasites by keeping the fish somewhat stressed all the time, and this
weakens the immune system. And at the low level of
salt generally recommended for these so-called benefits, there will be
no benefit that cannot be achieved solely with regular water changes
using a good conditioner.
Some concluding thoughts...
Using salt to increase water
Although plain aquarium/tonic
salt (sodium chloride) is sometimes suggested as a good way to increase
hardness and improve buffering, it in fact provides neither.
Marine salt mix, on the other hand, will raise the pH and
carbonate hardness quite significantly. But it also
raises the salinity, something most freshwater fish do not appreciate.
If you live in a soft water area and want to keep hard water
fish, using marine salt mix is not really a viable option.
Rift Valley cichlids, in particular, seem to be peculiarly
sensitive to salt, and elevated salinity levels have been identified as
one factor responsible for the dropsy-like disease known as Malawi Bloat
(Andrews, et al. 1988). 
Fish lore also has it that salt
is good for use with mollies, other livebearers and goldfish.
David Lass  notes that the vast majority of livebearers,
including mollies of all types and colors, and sailfins, come from the
Far East. They have been raised for generations in
water that is moderately hard, and of neutral pH.
These tropical fish are very far removed from the wild mollies that came
from brackish water. All of the sailfin and lyretail
mollies, balloon bellies, blacks, reds, and dalmations do fine without
salt. The same with goldfish. The
main confusion is that tropical fish need alkalinity.
Salt is just one part of alkalinity.
Although NaCl is not composed of
any truly "hard" ions, it does raise the total dissolved solids in the
water. This is not well tolerated by a number of
fish, especially true softwater fish from places like the Amazon River
basin, where there are very few electrolytes of any kind in the water.
Salt can have an unpredictable effect on softwater fish, since
there are no bodies of water in Nature which are naturally saline (high
in NaCl) while being very low in "true" hardness ion concentration
(calcium, magnesium, potassium, etc.). 
Salt and Plants:
When salt is added to the aquarium
water, the water inside the plant cells is less dense so it escapes
through the cells. The result is that the plant
literally dries out, and will wilt. I've so far been
unable to find a measurement of how much salt will be detrimental to
plants; all authorities I have found do note that some species are more
sensitive than others, and all recommend no salt in planted aquaria.
Domestic water softeners:
Domestic water softeners do not produce soft water in the
sense that aquarists mean. What domestic water
softeners do is remove the temporary hardness (such as carbonates) that
potentially furs up pipes and heaters by replacing it with permanent
hardness (such as chlorides) that does not. While you
can pass this softened water through a reverse-osmosis filter to remove
the permanent hardness as well, until you have done so, you shouldn't
consider the softened water as being suitable for soft water fish.
In fact, aquarists are divided
on whether the resulting softened water is safe for keeping fish at all.
The odd balance of minerals in softened water is not typical of
any of the environments from which tropical fish are collected.
While the chloride levels are much higher than those soft water
fish are adapted to, the levels of carbonate hardness are too low for
the health of hard water fishes like Rift Valley cichlids, goldfish, and
livebearers. So the safe approach is not to use it in
any aquarium, and instead draw water from the unsoftened drinking water
source in the kitchen. 
 Matt Clark, Practical
 Aldo Palmisano, Chemist,
U.S. Geological Survey Biological Resources Division, and an affiliate
of the University of Washington in Seattle.
 Stanley Weitzman, Lisa
Palmer, Naercio Menezes and John Burns, “Breeding and Rearing
Mimagoniates Species, Internally Fertilized Tetras,” Tropical Fish
Hobbyist, Volume XLIV, No. 12 (August 1996).
 Laura Muha, “The Skeptical
Fishkeeper” column in Tropical Fish Hobbyist, December 2006.
 Dr. Neale Monks, “Use and
Abuse of Salt and Epsom Salt in Freshwater Aquaria and Ponds,” Wet Web
 Stanley Weitzman, Lisa
Palmer, Naercio Menezes and John Burns, “Maintaining Tropical and
Subtropical Forest-Adapted Fishes,” Tropical Fish Hobbyist, Volume XLIV,
Nos. 10 and 11 (June and July 1996).
 Dr. Neale Monks, “A
Practical Approach to Freshwater Aquarium Water Chemistry,” Wet Web
 David A. Lass, “Using Salt
for Freshwater Aquarium Fish,” FishChannel.
Chen, Badman’s Tropical Fish.
 Mark E. Evans, “The Ins &
Outs of Osmosis,”: Tropical Fish Hobbyist, February 2004.