quote from RTR:
The final urban legend is 3e: The addition of NaCl at the same 1-tablespoon per 5 gallons as beneficial for livebearers. Here the pendulum swings closest to agreement with the myth. These fish as a group are native to estuarine environments where the waters are likely to be hard and alkaline at least, and may be brackish. Sodium chloride is matching their home, right? Well, maybe. If your water is moderately hard to hard (GH 8-12 or more, KH the same range), then some NaCl addition may be all that is needed, if anything is needed. If your water is soft and acid, you need buffering as much as the addition of minerals to the water, and NaCl alone will not be sufficient. You need to add crushed coral or aragonite plus salt, or just the use of marine mix rather than table salt alone. The marine mix contains the whole range of cations and anions (positively and negatively charged ions, such as Na+, Ca ++; Cl-, CO3--) found in the sea, and will be more than adequately buffered. The use of “sea salt” (a table grade of culinary salt from evaporation ponds of seawater) is not an adequate replacement. Brine in these evaporation ponds undergoes a large number of interesting reactions during the concentration process, so the final product does not match the composition or functional ability to support life that was present in the original seawater or in water made up from marine mix. It is fine for table use, inappropriate for the tank due to the significant differences from seawater.
So, is salt worth its salt? Definitely. It may be a part of the treatment of choice in certain conditions during an acute infection. It may even be part of the proper regimen for long-term maintenance of certain types of fish. Should it always be in your FW tank regardless of your fish or your water? IMHO and IME, absolutely not.
