Salt in the Freshwater Aquarium
Numbers in square brackets indicate a citation from the applicable reference works listed at the end.
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 pond).” 
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 this discussion.
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 eels. 
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 before excretion.
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. 
Osmoregulation 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 stability.
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 to death.
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.
Continued in Part Two
Salt -- Part Two
Some concluding thoughts...
Using salt to increase water hardness
Although plain aquarium/tonic salt (sodium chloride) is sometimes suggested as a good way to increase hardness and improve buffering, it in fact provides very little of either. 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, and these contribute to raise general hardness. 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 Fishkeeping. http://www.practicalfishkeeping.co.uk/content.php?sid=2850
 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 Media. http://www.wetwebmedia.com/FWSubWebindex/SaltUseFWArtNeale.htm
 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 Media.
 David A. Lass, “Using Salt for Freshwater Aquarium Fish,” FishChannel. http://www.fishchannel.com/fish-health/disease-prevention/salt-freshwater-fish.aspx
 Cecilia Chen, Badman’s Tropical Fish. http://www.badmanstropicalfish.com/articles/article5.html
 Mark E. Evans, “The Ins & Outs of Osmosis,”: Tropical Fish Hobbyist, February 2004.
I have always used aquarium salt in my fresh water tank ,
there are several benefits I found I gain from it ,
healthier fish live longer at-least in my experience , helps keep those pesky snails to a minimum but I haven't had a snail in 25 years ,
also helps keep your water clear , and helps maintain normal ph levels ,
keeps algae down to a minimum , plants also benefit from it
If salt is keeping your water clear, it is doing so by killing off bacteria. I can see no other explanation.
I would mention that many use it as a replacement for better maintenance; fewer water changes, etc. They seem to think this can work.
Snails are one of the aquarists best friends; they will help maintain a healthier aquarium.
As for the pH, I covered why salt cannot be used for this.
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