Thanks for the replies.
Wow though, they varied a lot

!
Anywhere from "none" to "100%"!
OK, sorry for the next question but I have to ask:
WHY is it "bad" to change a LOT of water?
I get my NSW from very clean locations, of course the ammonia, nitirites, nitrates etc. are unmeasurable.
Why then would it be BAD to introduce this excellent water into a reef tank?