What is chemically different about tea for hard water?
Hard water tea

By definition hard water is defined as water that has more than 0,6mg/l of dissolved CaCO3. Therefore soft water that doesn't have that much minerals has a greater dissolubility and can dissolve more tea. It can also dissolve the tea quicker. So the tea companies need to find what tea blend dissolves quickly and efficiently in hard water.


  A hard water blend needs lots of quality Assams and East African teas, with plenty of “guts” and strength. It needs flavour too, of course, but this flavour will be the rich, malty type that punches through the hard water. -https://allabouttea.co.uk/tea-news/hard-water-tea/ (as Bdrs commented)