I usually do it the next day cause I want to make sure the salt is fully mixed and other than the residu it’s fine. It just sucks cleaning the mixing container so much, it’s kind of disgustingMe too, with Red Sea Coral Pro I mix and add that day. It doesn't do well sitting for a week or more in mixing container for some reason. But my corals do love it so I mix and do the change 4-6 hours later