There's no need to use aquarium salt at all. Aquarium salt, sometimes called tonic salt, does not raise hardness, carbonate hardness, or pH. It does not buffer against anything. All aquarium salt does is raise salinity, and this can be useful in a very specific selection of situations. For example, a combination of head and salt can be used to treat whitespot more safely than formalin or copper. But apart from applications such as that, the routine addition of salt is obsolete and may even be harmful. Some aquarists consider to do so, but you'll be hard pressed to find any expert fishkeepers who do so.
If you have a pH around 6.5, but the carbonate hardness is 300 mg/l, something is very, very wrong. Carbonate hardness (which is more or less the same thing as alkalinity) is the ability of water to resist pH drops, i.e., acidification. Levels of carbonate hardness are measured in mg/l (= ppm) calcium carbonate. You can convert that amount into degrees KH by dividing by 17.83, so a carbonate hardness of 300 ppm is just under 17 degrees KH, which is a very, very high level of alkalinity. I'd expect a pH around 8. This is ideal for livebearers, Tanganyikan cichlids, brackish water fish, and various other fish that need hard, alkaline water. On the other hand it's far too hard for soft water fish like tetras, barbs, rasboras and angelfish.
My guess is that your tap water is rather unusual, perhaps because it's well water. In any case, you need to do a few things. First, draw some water from the tap, test the pH and carbonate hardness, and then test both again 12 hours later. See if the water chemistry changes. Often it does because of the gases and other substances in the tap water. Do also check you are using your test kits properly. Water that is soft and has a pH around 6.5 because it contains organic acids from decaying vegetation will also have a very low carbonate hardness, typically less than 3 degrees KH, i.e., less than 50 mg/l calcium carbonate.
In any case, salt has nothing to do with fixing this.
Cheers, Neale
PS. You may find my article on water chemistry useful, here:
http
/www.wetwebmedia.com/fwsubwebindex/fwh2oquality.htm
Hello, I just recently set up a freshwater aquarium, 20 gallons. I made the mistake of stocking my tank before it was cycled, and I had two fish deaths, but since then, I've taken good care of the other three fish (water changes, adding an airstone, testing the water, adding ammonia remover).
That's all besides the point, I just purchased freshwater aquarium salt, I would like to know if I should use it. My readings from about 5 days ago are pH is around 6.5, alkalinity over 300 ppm, Hardness is close to 0 ppm. Should I use it, and if so, how much?
Thanks!
