Since it's a new/quarantine tank, you could use a bunch of lake water to ease the transition for the fish. Just know that, as good of a way to ease acclimatization as it may be, it does increase the risk of introducing hitchhikers or parasites. But since you'll be deworming your fish anyway, it's a fine idea in my books.
The water from the bag? Naw, it will have his waste in it. Just pour it away.