I buy the Crysatal Salt sold in 40 pound bags at Lowe's for water softening. Mortons is sold for the same and costs about the same. Not sure what they put in the one that is said to remove iron and prevent scale. I would use the other for fish.
I have well water that comes from 300 feet down and is soft. I add Epson Salt and calcium chloride to bring both up to 50 ppm each in Mg and Ca ions. Epson salt is magnesium sulfate. Both magnesium and calcium ions are needed for life, so I add both to be sure there is enough present for my fish's health and for more rapid fry growth. If you have hard water, you might already have enough of both in your tap water.
I do not add salt (water softening salt) (sodium chloride) on a routine basis except to killifish tanks since salt helps prevent velvet in killifish and they are prone to get velvet compared to other fish. I do add salt to tanks which look like some of the fish are sick. I add a calculated teaspoon per gallon to these tanks with repeat additions for two more days. This usually stops whatever was happening. When I do the next water change, 50%, I do not add any salt, just the Epson and calcium chloride.
When I do add salt I add it as a saturated salt solution (SSS). You can prepare it by placing 3 or 4 cups of the water softening salt (crystals or pellets in a gallon jar and adding water to the top. Stir until no more salt dissolves and the clear liquid is saturated salt solution. A little cloudiness is OK. It's concentration does vary with temperature but not much in the range of 65 to 85 F, at least not enough to take into consideration when dosing a tank. Saturated salt solution is basically 27% salt and the rest water. So to dose a ten gallon tank with one tablespoon of salt (about 5 grams of salt) you want to add 50 grams of salt. 50 grams divided by 27% (concentration of salt in the saturated salt solution) is 187 grams of the SSS. This amounts to about three fourths of a cup of the SSS. Add the same amount on day two and repeat on day three for a ten gallon tank.
There are reasons not to add salt crystals directly to a tank with fish. It is better to add salt already dissolved in water.
SSS should not be used on fish and invertebrates known to be sensitive to salt.
The full article on Salt Uses in the Aquarium can be read on my website at aquaticlifefarm.com
