are u talking bout during water change, or top off due to evaporation? ur never supposed to add salt to ur tank, always pre mix it and let it age a day or two. if ur topping off water due to evaporation, then u should only be adding fresh, di or ro water. if ur adding salt water as ur top off water, ur just adding more salt and raising ur salinity, because water is loss during evaporation, not the salt. if ur talking bout water for water changes, get like a new 5 gallon bucket, then add salt to di or ro water, and mix it, test with refractometer or hydrometer, and let it age a day or two.