Why would an aquarist put salt in a freshwater aquarium? Aquarium salt has been used for decades as a sort of "tonic" for freshwater fish. While it's hard to put a finger on just why salt is beneficial, many aquarists have noticed that certain fish simply do better with a small amount of aquarium salt added to their tanks.
What are the advantages of using aquarium salt? First, some tap water sources are very low in dissolved salts compared to certain fish-collecting or fish-raising areas, and the addition of aquarium salt might simply make the fish feel more "at home". Second, salt provides replacement sodium and chloride ions that stressed or sick fish need. Third, salt may inhibit the fishes' uptake of toxic chemicals like nitrite. Finally, salt inhibits parasites (for example, Chilodonella cyprini) that are sometimes difficult to diagnose or treat.
Do all fish appear to benefit from adding salt? Certain fish, like livebearers and of course brackish water specimens (those that inhabit waters with significant salt levels) like monos, scats, puffers and fiddler crabs seem to have far better survival rates and fewer disease problems when kept with salt. Many others, like most barbs and tetras, seem more indifferent. A few, notably Corydoras catfish, are said to have very low tolerance for salt, although our experience has been that they do quite well at the salt levels we use.
How much salt should be added - and what type - and how often? A tablespoon for every five gallons of water works well for us. Use either "Aquarium Salt" from the aquarium shop or Kosher or canning salt from the grocery store; the idea is to get just salt, with no additives like iodine or potassium. Salt should be added with the initial setup and only replaced thereafter with water changes (e.g., if you change 10 gallons of water in a 30 gallos of water in a 30 gallon tank, add 2 tablespoons of salt).
Does Aquatics Unlimited use salt in it's display tanks? We've used a tablespoon per five gallons in virtually all our freshwater tanks for over 25 years with no apparent problems, and have noticed benefits, particularly in dealing with swordtails, mollies and other livebearers . We've recently discontinued its use on angelfish and discus, to see if it has any effect on their acclimation (so far it seems to have made little difference either way).
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