I apparently know a little more than you since I don't have to ask questions about water chemistry (ah hem)..I never said a person would use salt to soften their water.
That aside, I think I also explained this defination once before on another thread, but it does bear repeating apparently.
In chemistry and scientific terms, the words salt and mineral are interchangable and frequently even redundently used together. Haven't you ever heard of mineral salts? All salts are minerals.
Having the right quanity of mineral/salts in their water is just as important to fish as to us getting the right amount of zinc, iron, and electrolytes in our diets. We don't think about it until we have a defeciencies in them. Same with fish. Fish kept in pure RO water (water that contains no minerals or salts of any kind) die just as we would die if we only drank RO water as the sole source of minerals in our diet. Even the bottled water we drink has minerals in it, often artificially replaced by the manufacturer.
And that what aquarist must do to recreate the right balance of mineral/salts in their tanks for their fish. Soft freshwater has very little mineral/salt content in it, but it must have =some= in there to provide the fish with what they need for proper physiological funtioning. The fish that live in soft freshwater are adapted to low mineral contents. To achieve soft water, the old-school method of doing so was to filter it though peat. Peat filtering would move a lot of the mineral content of the water, lower the pH, yet leave behind enough minerals to give the fish the balance they needed. The new-school method is to use RO water (water that has been completely de-salted/de-mineralized) and then artifically add the proper salt/mineral content back to it to achieve the balance the fish need.
In the instance where you are using RO water to supply the tank, then yes, you would have to add salts back to the soft freshwater of the tank.
Hard freshwater has a lot of minerals in it. While all minerals are salts and all salts are minerals, not all salts are NaCl. Hard freshwater has a high concentration of minerals such as calcium carbonate and other carbonate/bicarbonate salts, but doesn't necessarily have a high concentration of sodium chloride. African Cichlid salts increase the mineral content of the water they are added to in the correct proportions for those fish without increasing the concentration of NaCl to where it would qualitify as saltwater (defined by NaCl concentration).
Technically it is possible to have saltwater without any NaCl in it, but that is just a matter of scientific semantics.
Saltwater, because of the inherited high mineral concentration from NaCl is hard water. But just because it has high concentrations of NaCl does not make that salt water SEAwater. SEAwater, which is what most marine aquarist must artificially create in their tanks, contain more varieties of minerals other than NaCl salts, including gold and silver, zinc, iron, lead, and copper, in the ratios that they need to be to support life.
Fish kept in strait NaCl saltwater would die just as much as fish kept in strait RO water would.
If you have questions about buffers, go ask your high school chemistry teacher.
~~Colesea
