There are definitely ways to soften water and lower pH but I have never tried. I think most fish are good in water that is moderately soft to moderately hard with a pH of 6.5 - 7.5. Unfortunately, your water seems to be very hard and alkaline. Filtering through peat moss will lower the hardness and pH. I don't have any experience using it so not sure what it entails exactly. If it's as simple as putting it in your filter and replacing it monthly, maybe that would be worth it for you. As driftwood decomposes, it will also lower the pH and soften the water. Hopefully, others who have done this will chime in. If you would rather not have to worry about your water parameters, you could go with a Firemouth. A single specimen with a group of sword tails would work in the 55 for sure. Or if you don't care about keeping fish from the same region together there are many different varieties of Australian rainbows that would work as well.