I want to further add to what John says above…..natural sea water, not an issue. However, I would strongly suggest not doing this extremely large water change with the powdered salts most (if not all) of us use. These “synthetic” salts contain ammonia to help in dissolution. With small water changes, this is not an issue. I would be concerned about this ammonia with a large water change.
Paul you seem to be filled with a ton of info so I have to ask what salt do you use and if you say you make your own I think my head will explode