You could try a combination of tap water and ro water (That's what i do). There are calculators out there to work out the ratio of tap to ro to attain the level of hardness required (try www.practicalfishkeeping.co.uk)
RO is pretty cheap to buy. My local LFS sells 25litres for £1.75. I think as long as you haven't got an exceptionally large tank, say 350 litres plus, buying ro and mixing with tap water isn't too bad.
You can also try boiling your water to reduce hardness, but this is quite a chore as it takes ages to boil the right amount.
Russ