That depends on your tap water parameters (pH and hardness). You can get a good idea by going to your supplier's web site and looking for the page on water quality. You should find you can put in your postcode and get a water quality report including those. Post them here when you've done that and we'll make some suggestions for you.
There should be a specific section on the water supplier's website that gives actual numeric water hardness readings in CaCO3 or German degrees hardness - if you can find those results and let us know the exact readings, that will help us to work out which fish species' requirements match those of your tapwater.
As a "starter for ten", though, possibilities for harder water might include platies, endlers, guppies - but we can provide further suggestions once we know the above.