What you need to do, is match the salinity and the temperature, to exactly what it is like inside of the tank already. If not exactly, VERY close. This will minimize stress on your inhabitants. Pouring in water that is cold.. and even 2 degrees less in salinity could be the determining factor as to whether or not your fish is stressed or it's happy. I did it once, and my clownfish bugged out. Next water change i matched the temperature and the salinity exactly as it was in the tank, and when i poured the water in, they came out and played in the water and ate all the bubbles. So really, it does make a big difference to match the waters up accordingly. Also, 24 hours is usually a good time to let the water sit. You need to be able to visually see that there is nothing left in the water. And if you add more salt because the salinity wasn't were it needed to be, that's 24 hours again. You need to be as safe as possible when it comes to mixing salt, because salt is caustic to livestock and can burn and damage their gills and other parts.