"For nitrite poisoning salt can impart protection to the fishes. This happens only if the salt content is such that the chloride ion’s concentration is about 30 times that of the nitrite ion concentration in the water. Typically, nitrite becomes toxic at about 0.1 mg
/L. This means that the chloride ion concentration would have to be at least 3.0 mg
/L. This concentration translates into one of about 5.0 mg
/L of salt (NaCl is 60.66% chloride, Cl
-); this is equivalent to 18.7 mg
/gallon. A teaspoon of table salt is about 5.5 grams (or 5,500 mg
); a teaspoon of table salt would be sufficient to protect fishes living in approximately 294 gallons of water ! A standard treatment of SeaChem Prime will typically provide enough protection."
That is from JmcPeak's link Blue Betta USA - The Use of salt
I wish it had references though, I'm hesitant to blithely believe things.