Salt interferes with osmoregulation, and is also an irritant to freshwater fish.
The "tonic" effect often talked about is created by the interference with Osmoregulation. Over time FW fish that are exposed to slat will become weaker and unable to cope with normal attacks on their immune systems. They generally do not live as long as FW fish which are kept in Freshwater.
The claims of Slime coat production ore of course real, since the fish's natural response to any irritant is to produce more slime coat, they will inevitably do so when exposed to salt. Irritant= reaction. battery acid or gasoline will get you similar reactions
As far as Guppies, This is agreat myth in the hobby. And it goes something like this:
"livebearers are mostly brackish so they need salt" " liveberers need salt to thrive and reproduce well" Or "Mollies are brackish fish in the wild so you have to add salt or they will die"
HEre are the facts: Guppies, mollies, platy's, swordtails, killiefish, endlers etc.etc. are all freshwater fish. of these mollies are the only ones commonly found in the wild in brackish or marine water however they are still predominantly freshwater fish which come from freshwater origins. Additionally, most of the fish in the hobby are tank raised and bred in freshwater, so even if the origin of the original stock has some question to it, the fish sold in the LFS are Freshwater.
Next fact on this thought. Adding salt does not in any way come close to duplicating Brackish conditions. Seawater contains a huge list of minerals, and sodium and chloride neither one are the most prevalent. There is a good bit of salt in Seawater, but there is also a good bit of many other things necessary to the equation.
With livebearers, hard water is usually desired, because these fish do tend to thrive in high calcium high magnesium environments. This also means High TDS (Total Dissolved solids) levels. Since most livebearers are hardy, Most will tolerate salt moreso than other fish might, and since they thrive in High TDS water, the salt sometimes appears to have a positive effect. Salt does in fact raise TDS levels.
For any livebearer except mollies, Whatever your tap provides is awesome. you can't keep them from thriving and breeding, and they adapt to anything from soft to hard water. Mollies can sometimes be a little finicky with really soft water, but will still thrive, they just may not breed as much. For mollies I reccomend some attention to magnesium and calcium and you will see much more breeding activity.
In this whole discussion it is imprtant also to seperate Salt (NaCl) from mineral salts. For instance Epsom salt is MgSo4 (Magnesium Sulfate) and is a great thing to use to increase magnesoium levels.
Many Cichlid Salts Sold in the industry are a combination of mineral salts which produce very hard water for african cichlids. There is a balanced or correct amount of sadium and chloride in these products, but not much because there isn't much in the rift lakes in reality. Cichlid salts and cichlid substrates can be very useful for livebearers but once agian they are seldom needed and not often worth the trouble.
Just for a perspective, The saltiest of the rift lakes is Tanganyika, and the combined Sodium and chloride levels equate to less than 1/4 teaspoon per gallon. Doc Wellfish reccomends 3 teaspoons of salt per five gallons of water. so roughly 11 times the amount found in the saltiest freshwater in the world.
The next part of this argument is always the claim that aquarium salt is somehow magically better than table salt. The marketing department did great things on this one. I know people who use aquarium salt and then turn around and add iodine spplement for their shrimp :thud: :thud:
The trace amounts of anti-caking agents and iodine found in table salt will harm nothing in your tank. so in addition to not needing the product long term, when you do need it short term (Ich or nitrite poisoning) you still don't need to buy the high dollar salt from the LFS. mortons Iodized table salt is my product of choice for the rare occasion I do actually need salt.
I never expose my fish to salt unnecessarily or for any extended period of time.
For more information here is a great article.
Salt