Adding salt to your tank is just not based on science. In fact, it goes against many of the principals of aquatic biology as we know them. Some folks believe that "salt helps fish regulate their osmotic balance" .. this is patently rediculous! All fish are different in this regard, depending on where they live. Some species are native to "soft water/low dissolved solids" habitats while others inhabit hard and alkaline biotopes with high salt concentrations inherant in the water. The species inhabiting these very different environments have developed metabolic processes which take advantage of the specific chemistry of that water. To add salt in the misguided attempt to "help regulate osmotic balance" may actually upset that very balance you are erroneously trying to "help". Your fish are much better served by leaving them alone and allowing them to regulate their own osmotic balance ... something they have been doing without your help for thousands of years. Some folks will tell you that salt is a tonic for fish when it is kept in fish tanks at moderate levels. Poppycock! If your fish are the most commonly kept community fish such as tetras, corys, angels, rasboras and most anabantids, these are fish largely from soft, acid, low-TDS (total dissolved solids) waters. The average tap water in the U.S and Europe is at least moderately hard and alkaline and is certainly not improved by increasing the "salt" concentration ... that very thing in which your water already differs most from the natural waters of these fish. Certainly many of these fish adapt well to our local water conditions and I am an advocate of adapting non-breeding fish to local conditions. This is far better for both fish and keeper than constantly battling see-sawing water parameters. But, acclimation to your tap water is one thing .... making your water worse than it already is however, is quite another. Then there's the prophylactic use of salt to avoid common pathogens and parasites of fish by keeping some level of salt in the tank ... more baloney. True, the use of salt, usually accompanied by increased temperature, is an effective treatment for one of the most common ectoparasite which beginning aquarists encounter ... Ichthyophthirius multifiliis, or Ich. However, only the free-swimming form of this parasite is treatable. Neither the fish-embedded growth phase, nor the encysted multiplication stage that rests on or in the substrate is readily attacked by salt or any other medication. Thus prophylactic treatment is useless as it makes little sense to treat a non-infected tank on a chronic basis. Another falacy is the addition of NaCl for livebearers. These fish as a group are native to estuarine environments where the waters are likely to be hard and alkaline at least and may even be brackish. The addition of sodium chloride (i.e, "salt") does little to match these species native waters since salt comprises only a portion of the total "salt" concentration. The more significant elements are calcium, magnesium, potassium, boron and silicate salts. It's important to note that the term "salt" isn't limited to sodium chloride. Calcium and Magnesium salts of carbonate, silicate, phosphate and borate are actually more important to the biology of brackish water fish. If your water is at least moderately hard (GH and KH 8-12) then the addition of some NaCl will do no harm. On the other hand if your water is soft and acid the addition of sodium salt will do little good. You need increased buffering capacity as much as the addition of minerals to the water and NaCl alone does absolutely nothing in this regard. You will need to add crushed coral or aragonite, both of which are primarily calcium carbonate salts, in addition to 'salt' as sodium chloride or better still, use marine salt mix which contains the whole range of cations and anions (positively and negatively charged ions, such as Na+, Ca ++; Cl-, CO3--) found in the sea. This will adequately buffer your water and more closely match the native waters of most live bearers in question. Salt does have other limited uses. It temporarily reduces the effect of nitrite toxicity at 0.1-0.3 % -- the chloride ion counteracting the nitrogen blockage of oxygen uptake. Salt is useful for the erradication of hydra at 0.3-0.5 % for five days. Salt will remove leeches from pond fish as a 3.0 % bath for 15 minutes. Salt will mitigate the affects of ulcer disease in cold water fish as a 1.0 % addition to the tank water by temporarily reducing osmotic stress. Beyond these few 'uses of salt' ... it has no place in your aquarium. Let's talk about some other aspects of "salt", in particular .. the hardness or softness of your water. The term 'hardness' arose historically because water with higher levels of Ca and Mg are more difficult to use for washing clothes - - - it is harder to produce a lather from soap (or from detergent), hence “hard” water. General Hardness (GH) is a measure of the concentration of calcium and magnesium ions (Ca++, Mg++) in your water. Common ion exchange resins, both those used as pillows in tank filters and in bulk in household water “softeners” most typically exchange (Na+) for (Ca++) and (Mg++). The nature of resin chemistry dictates that the charges must balance, thus two (Na+) ions must be added to the water for each (Ca++) or (Mg++) removed. The resulting water will be 'softer' by the laundry definition but not by fish standards since the water now contains more total ions than it did before the softener. The total dissolved solids (TDS) in the water are higher than that with which we started despite the fact that the GH test shows lower readings. The importance of this for fish-keepers is this: Those fish which we call "soft water fish" do not want "soft water" as we define it since fish don’t do laundry. They DO want water with low TDS which would include low GH as we measure it and also low Na+ and Cl-. The Amazonian fish and some of the SE Asians and riverine African cichlids come from waters with low TDS .. yes, the water would read low in GH but would be low in sodium as well. So what's the bottom line? If you want lower TDS water, you must use peat extraction, de-ionization (DI), or reverse osmosis (RO). Otherwise you do not have appropriate water for "soft water fish".