Freshwater fish do come into contact with salt, though the Amazon is one the exceptions where the salt content is generally much lower than it is in other freshwater bodies. Many claim that Corydoras and Malawi cichlids tolerate salt badly, however, research shows that many fish from soft, acidic water will tolerate salt.
That being said, salt should not be used regularly. Salt has some valid uses in the aquarium for treating health problems, water quality problems and minimising stress, especially during transport, however, most of these work by salt's effect on the osmoregulation of freshwater fish.
The Use of salt (sodium chloride, NaCl) in the freshwater aquarium or pond.
What is salt?
In the broadest sense a “salt” is the chemical compound formed when an acid reacts with a base. There are many “salts” in our everyday lives: baking soda, or sodium bicarbonate, NaHCO3, and washing soda, soda ash or sodium carbonate, NaCO3, are two examples. The more familiar table salt or sodium chloride, NaCl, is a salt that is formed when hydrochloric acid (a.k.a. muriatic acid) reacts with sodium hydroxide (a.k.a. lye). Common salt exists in several forms and most of these are found in the modern home. There is the stuff found in the salt shaker in the dining table, there is the rock salt we use to melt ice and snow, and there is the water softener salt we use to recharge our home water softener zeolites.
Are all forms of common salt essentially the same?
The short answer is “yes”. For all practical purposes these different forms of household salt are the same. The basic differences are found in the purity and the additives. Rock salt, basically, is just the material as it comes from the salt mines or evaporation ponds. It contains greater or lesser amounts of impurities and is not suitable for human ingestion. Water softener salt is a purified form that is usually pressed into pellets. Except for its physical form it would be suitable for use in cooking; it is also a bit pricey to be using as an ice-melt. Table salt comes in two basic formulas; iodized and non-iodized. The latter is used where natural iodine content in the drinking water and other food sources may not be sufficient to prevent human goiter (a disease condition of the thyroid gland). Table salts also contain one or more additives that prevent them from caking, and therefore, free flowing, in a humid atmosphere. “When it rains it pours.”
Are the additives in table salt harmful in aquariums or ponds?
The short answer is “no”. Neither the small percentage of iodides nor the anti-caking additives can be considered to be dangerous when the salt is used in ponds or aquariums. There is no valid reason to use only non-iodized salt yet this is always a hot topic for discussion and argument among aquarium hobbyists. In reality, the minuscule amount of iodide that would end up in a treated pond or aquarium is likely to be beneficial (as a potential source of essential iodine for both certain plants and animals) and certainly not harmful. Fishes can suffer from goiter (= iodine deficiency) also.
How useful is salt used in aquariums and ponds?
From the number of aquarium hobbyists and pondkeepers who use salt one would have to assume that it is one of the best, if not the best, treatment to use for all kinds of known and unknown diseases. The reality is quite different.
It is quite possible that the unwarranted reliance upon salt treatments has resulted in more fish losses than the diseases themselves. This is because there is a common misunderstanding among aquarists and pondkeepers that salt is a good disinfectant, antibacterial, antifungal and/or antiprotozoal drug. At the concentrations commonly used in aquariums and ponds it is none of these things. As a reliable disease treatment and/or preventative salt is essentially useless.
For what is salt useful?
Primarily, salt can be used for two different, but related problems. The first is the treatment of nitrite, NO2-, poisoning, and the second is the treatment of osmoregulatory stress. In both cases it is the property of sodium and chloride ions to be transported across the gill membranes, from the water, and into the blood of the fishes. Osmoregulatory stress can result when fishes are transported or when they are removed from water of one osmotic pressure and placed into one which is significantly different. In general aquarium and pond practice, where one is working only with freshwater fishes, the problems associated with osmoregulatory stress are essentially nil and therefore of little concern.
For nitrite poisoning salt can impart protection to the fishes. This happens only if the salt content is such that the chloride ion’s concentration is about 30 times that of the nitrite ion concentration in the water. Typically, nitrite becomes toxic at about 0.1 mg/L. This means that the chloride ion concentration would have to be at least 3.0 mg/L. This concentration translates into one of about 5.0 mg/L of salt (NaCl is 60.66% chloride, Cl-); this is equivalent to 18.7 mg/gallon. A teaspoon of table salt is about 5.5 grams (or 5,500 mg); a teaspoon of table salt would be sufficient to protect fishes living in approximately 294 gallons of water ! A standard treatment of SeaChem Prime will typically provide enough protection.
For simple osmoregulatory stress protection, on an indefinite basis, one can use 1 to 3 mg/L of salt. This would be equivalent to one teaspoon of salt added to 1,453 to 484 gallons of water!
What about disease treatment with salt?
In his book, Fish Medicine (W.B. Saunders Company, 1992), Michael Stoskopf lists salt at a concentration of 22 mg/L (= 83.27 mg/gallon), as a dip, for 30 minutes to control fungal infections and protozoal infestations (specifically Epistylis sp.). This treatment level is equivalent to 1 teaspoon per 66 gallons of water! Stoskopf, lists no other treatment uses, and to use salt in place of reliable and safe treatments like Rid-Ich+ for treating diseases, like ich, is simply not justified.
To use salt at higher levels (e.g. 1 teaspoon per gallon) than those indicated by the literature is also not justified. Higher levels, especially on freshwater fishes such as characins (tetras), cyprinids (goldfish and koi) and catfishes, will act as an irritant and thereby stress the fishes. Fishes such as livebearers and cichlids can, likely, tolerate much higher levels of salt (some species of livebearers and cichlids actually enter seawater).
Before using salt in an aquarium or pond:
- 1) Consider the fact that salt is a very poor substitute for more specific drugs, medications or preventatives.
2) Salt can be highly irritating to freshwater fishes, especially cyprinids (goldfish & koi), catfishes, and characins (tetras).
3) If salt is indicated (such as for nitrite poisoning) one can use either iodized or non-iodized with equal safety.
4) If salt is used to ease osmotic stress, use it only once, and then get rid of it by doing regular, partial water changes on the pond or aquarium.
5) Do not use salt in place of proper water conditioners and shipping aids such as AmQuel, NovAqua, PolyAqua, Prime or Ship Right.
6) Salt is not a disinfectant. For proper disinfection of nets and pond and aquarium equipment use SanAqua or Net-Dip.
