12-11-2012, 12:39 PM
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As someone else already said, I believe, the context of salt use is important. The term salt may refer to NaCl, or it may refer to a mix of mineral "salts" , including sodium, potassium, etc. dissolved in water. I realize that's kind of a "duh" statement, but it matters to the fish. Depending on where a fish is native to, the amount of different minerals in the water where the fish evolved vary. Some fish naturally tolerate a significant amount of the mineral "salts" in their water. Others, particularly some South American fishes, don't tolerate salt well at all.
Euryhaline fish (fish that have a large tolerance to varying degrees of salt) may actually require salt in the tank (examples include the brackish water fish). Mollies come to mind. They do better long-term with some salt in the tank and you can actually acclimate them (slowly) from a freshwater tank to a marine tank.
Bottom line, do I add salt to any of my FW tanks? Nope, but I might try it as an ick treatment if other methods failed me. Salt's not necessarily good for FW plants and all my tanks are planted!