For treating skin parasites and free-swimming Ich swarmers, as a half-hour dip to make gill flukes let go before quarantine-- salt has therapeutic uses. But as a teaspoon to five gallons permanently, on an "it-couldn't-hurt" basis... no.
Today's NY Times had a piece about the hundred-year battle against the "catch a chill going out with your hair damp" common cold theory, that no amount of genuine science can dislodge. I kept thinking of all the people adding hot water to keep their water changes within 2 degrees, "not to get Ich."