Some hobbyists religiously use salt (sodium chloride) in freshwater aquarium set-ups. The claim is a noticeable health improvement of certain fish. Further benefits include the ease of stress, reducing osmotic pressure, inhibition of nitrite
uptake, promoting the slime coat, and helping in healing wounds. The
salt recommended should be free of additives such as iodine.
It is claimed that salt is safe and should be used as a preventive measure against various parasitic infestations. Salt is also said to cure various diseases.
The recommended quantity ranges from 1 tablespoon per gallon to 1 tablespoon per 5 gallons.
At first blush, the claims do not sound bad.
Salt (sodium chloride/ table salt) does in fact have a direct connection to osmotic pressure. To explain this, picture a fish in an aquarium. The
internal density of fish is greater then that of the water (fish contain salt in form of sodium and chloride ions transported by the blood). Incoming water tries to dilute their bodies to equal both sides, the inside of the fish and the water outside.
pressure can be best described as the water trying to dilute the fish's
body until both sides are equal. Freshwater fish therefore have to
constantly eliminate the water - mainly through respiration and urine.
same applies to saltwater species, but in this case the roles are
reversed. Saltwater fish have to "drink" water in order to survive.
stress can occur during the transport of the fish, but is taken care of
by stress protecting additives right from the beginning. Other than
that, osmotic pressure is essentially non-existent and needn't be of
the osmotic pressure issue, should salt be considered, for whatever
reason, one teaspoon would be sufficient to treat about 500 Gallons of
It is also claimed that salt should be used in the prevention of nitrate poisoning. Theoretically, this is a true statement.
Let's assume your tank is brand new and cycling, or the beneficial bacteria
are adjusting to a change in tank inhabitants, or worst case, you
killed some bacteria colonies using antibiotics to nuke the small algae glancing at you.
Salt actually can be used to prevent nitrite poisoning, if the chloride ions are 30 times the concentration of nitrite ions.
reaches a toxic level at about 0.1 ppm, which would require about 3 ppm
of chloride ions. Depending on the salt (sodium chloride) used, it
might translate to about 5 ppm (given that common salt has a chloride
concentration of 60%) to ease possible nitrite poisoning. This in mind,
one teaspoon of salt would be sufficient to provide this effect for a
300 Gallon tank.
brief summary, 1 teaspoon per 300 Gallons will do as described above.
Table salt does contain iodine and anti caking additives (to prevent
the salt from clumping together). Iodine is essential for certain
plants and animals, and definitely of no concern, considering the low
amount of salt and the low concentration of iodine added to the salt.
Iodine at this concentration should be rather beneficial instead.
For minor cases of flukes, fungus, external parasites, etc., a salt bath can assist the fish in healing. This is partly related to the benefits of osmotic pressure regulation.
is relieved and the organism can fight off diseases easier which aides
in the recovery. The concentration should be 4 teaspoons per Gallon and
the duration of the bath about 30 minutes. This bath will also
stimulate the protective slime coat, which will further enhance the
fish's ability to cope with the disease.
(one celled parasites) on skin, gills and fins can effectively be
removed by a salt bath. For the record, some fish do not respond well
to a salt bath (i.e., some barbs, tetras, catfish and koi). A heavy
concentration can make them loose their equilibrium and they simply
"roll over". At this point the fish has to be moved to clear water very
Some other considerations should be mentioned, before drawing a conclusion:
does not evaporate, it can only be removed by water changes and plants
will not survive higher concentrations. The reason is similar to what
we can observe with fish that cannot survive higher salt
concentrations. Once again osmosis is the reason. Freshwater naturally
moves from an environment with a low salt concentration, (inside the
plant or animal) to one with a higher salt concentration (the water).
As a consequence the diversity of plants and animals decrease. This is
because they cannot keep the water and salt content of their bodies at
the right concentrations for them to survive this environment. The
lethal point for plants is reached at about 1,000 mg/l of salt. One
teaspoon of salt equals approximately 5,500 mg. That is to say, 1 teaspoon of salt per gallon reaches, and even exceeds, the lethal point for plants.
does interfere 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.
low beneficial amount of salt, mentioned above will not have any
benefits in addition to water conditioners and/or stress coats already
used for water treatments.
It is good to know about the benefits of salt and the understanding of the mechanisms involved. It comes in handy, should the nitrites get out of control or as possible treatment for parasites (salt bath). A first aid kid, for sure.
last, disease prevention and cure. This is largely if not mainly based
on enhancing the slime coat or regulatory osmotic control, but again
stress protecting additives and water conditioners have the same effect.
prevention? Yes - in theory. But it is not justified. The long-term use
of salt in the aquarium will have more negative aspects then benefits.
Use of salt as a first aid tool should be determined on an individual
basis, as there are no real guidelines on how to use it safely and
The immediate threat posed by salt to both fish and plants is far greater than that of any potential long-term benefits that may, or many not, be gained by its use.
Because of the potential hazards to the health of your fish and plants, we do not recommend the use of salt in freshwater aquariums. However, its use or not is a matter that only you can decide. The intent
of this article is to provide an objective look of the effects of salt and should
only be viewed as a discussion about the topic.