How to brew tasty tea with hard water
You can also get a water filter that is meant to help with this sort of thing.

Here in the UK one of the most common brands is Brita, you simply put a couple of litres in a jug which has a filter in it, after filtration use it to make your tea.

I've not used it myself however so can't comment on its effectiveness.

You can also get these filters built into your taps.