Copper stain in bath from new hot water cylinder

Hello. Two months ago, we had a new hot water cylinder fitted after our old one started leaking. Since then, we've seen a blue ring appearing in the bath, presumably from copper corrosion. Is that normal with a new tank or is it something that needs to be looked at? In case it's relevant, although we live in a hard water area our water (including hot water) is softened. Many thanks for your help.