Three teaspoons per gallon or 0.3%.
First dose-One teaspoon per gallon.
Second dose-Repeat first dosage in 12 hours.
Third dose-Repeat the same dosage in another 12 hours right after second dosage.
Dissolve salt thoroughly in a cup of tank water before adding as a precaution. And every new fish as a rule (and it should rather be set on stone IMO) should be quarantined for at least 3-4 weeks. Patience will pay you off eventually. The longer a fish is quarantined, the better as it allows you to observe further for unusual symptoms. Some diseases do not come out until later on.
Ich makes a good floodgate for bacterial infection as the protozoans pierce through the skin thus exposing the blood vessels to aeromonas. Aeromonas bacteria is always present in the tank so be on the lookout for hemorrhaging, secondary phenomenon to ich and any parasitic infection.