Ppl with the knowledge of fish keeping, would not use RAW tap water, some will use chemicals to rid it of chlorine.
I for one, often do 50% or more water changes, BUT, using water that has been standing after being squirted into the container , for at least three days, often over a week. Most of these changes this size, is done on my baby tanks or containers.
If the fish are use to this, they will not react.
Alan