Any calcium carbonate present in your tank will tend to raise both the carbonate hardness and the water's pH. One of the most readily available forms is simple limestone. A few limestone rocks in the tank will definitely harden your water and raise your pH. As most other livebearer keepers can attest, high mineral content and a high pH is really a positive thing for any of the more common livebearers. There are some livebearers that do best with low mineral content water but they are far less common in the hobby than things like guppies. I have found, as a livebearer specialist, that almost any readily available livebearer will thrive in the high pH and high TDS regime. BTW I consider things like goodeids to be readily available because a dedicated club participant does not find most goodeids to be too hard to find. I have some nice blue goodeids, A. towerii, in my tanks and they fall right in line with the rest of my fish, even though many people would have a hard time finding a breeding colony of them. Any dedicated hobbyist can find them along with other livebearers. I absolutely admit they cannot be found in a pet shop, any pet shop, but they are not all that hard to find in hobbyist locations. I got mine at the ALA convention last year.