The main theme of this article is answering the question of whether or not aquarium salt is the same as marine salt, and if they are different, how. Sources of information will be listed below for further reading. I would like to also reccomend that anyone interested in understanding these issues and other issues visit the Skeptical Aquarist and the AquariumWiki.
Aquarium Salt vs. Marine Salt
CONCLUSION
Aquarium Salt and marine salt can both be useful but are not interchangeable in MOST instances and should be considered two completely different substances with different applications. Aquarium Salt can be used carefully, in some situations, to treat Ich or to lower Nitrite stress, while marine salt sustains marine fish, inverts and bacteria.
SUMMARY
i) Aquarium Salt (there is no legal definition of this, it's an arbitrary term) and marine salt may have many similarities but the small differences completely change the useage of each product.
ii) What is often sold as "Aquarium Salt" is NaCl (Sodium choride). Marine salt is also Sodium chloride, however, marine salt is different in that it includes calcium and magnesium ions which will greatly effect the alkalinity of the water it's added to.
iii) Using "Aquarium Salt" in a reef tank will increase the NaCl levels however does not have the calcium and magnesium carbonates necesassry to sustain marine life.
iv) Using marine salt in a freshwater tank would have many of the benefits of "Aquarium Salt" but would also cause large shifts in alkalinity, PH, TDS and GH, which are fatal to most fish
v) "Aquarium Salt" has many obvious pros and cons:
PROS
-The freeswimming form of Ich cannot live in water with high concentrations of salt
-Restores electorlytes
-Reduces "brown blood" syndrome after shipment or during times of Nitrite spikes.
CONS
-Increases TDS readings; many tropical fish (see below) prefer low TDS readings especially when breeding.
-If not predissolved, salt will burn fish.
vi) When deciding whether to use a simple NaCl product vs a marine salt for brackish fish, consider that many brackish organisms live in waters close to the ocean, which has a high alkalinity and GH. Thus, many of these organisms naturally operate with no osmoregualation stress in water which is higher in alkalinity and dissolved caclium and magnesium ions. Thus, marine salt is the better option as "Aquarium Salt" does not increase alkalinity or GH and doesn't provide the ideal habitat for the fish.
vii) At a certain salinity (certain dissolved substances, whereas TDS is ALL dissolved substances), which RTR states is 1.010 - 1.012, all bacteria in the tank will die and a saline-tolerant version will begin to grow. Therefor, adding marine salt at higher than those levels will cause a tank to re-cycle.
viii) Some fish can handle salt better than others - fish which live in waters with high GH, KH and PH such as African Lake Cichlids, Marine Fish, Livebearers and brackish fish will all thrive. However, amazonian fish will show signs of osmoregulatory stress if salt is added - too high levels can lead to death of fish and also to the eggs (hence why breeders take PH, GH and KH so seriously).
RESEARCH
NaCl vs. Marine Salt
"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." - wetman
What is aquarium salt?
"There is no such thing as "Aquarium Salt". It is whatever any given manufacturer packages and labels as such. It may be routine iodized table salt, it may be unpurified and supplemented mined salt (but usually not as unsupplemented salt (NaCl) tends to fuse into a block from humidity, nobody buys such twice), or any grade of purified salt. Some is sea salt from evaporation ponds. In other words it is a pig in a poke. There is no legal definition of or for aquarium salt."- wetman
Salt and bacteria
"Any salt added at and above specific gravity 1.010 -1.012 will kill FW nitrification bacteria. Smaller additions may do a partial wipe-out but not likely total unless done fast enough to osmotic shock the bacteria - which will osmotic shock the fish even quicker. If such additions are by table salt or pure NaCl, it will not support marine life of fish, inverts, and bacteria. Only marine mix (a very complex mixture of a large number of different chemicals) will do that. "Aquarium salt" will not support marine life, nor will table salt, rock salt, or sea salt."- Robert T. Ricketts
Salt and tolerance
"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; small tetras are less tolerant. But there are many exceptions to this very broad rule. Be aware of the salt tolerance of your particular fish."- wetman
Salt as a preventative and treatment
"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.
Aquarium Salt as a treatment: 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 TFHinterview a decade ago. 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.
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."- wetman
SOURCES
wetman via The Skeptical Aquarist
Robert T. Ricketts via The Aquarium Wiki "the Salt of the Earth, the Salt from the Sea..."
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