Ohh!! You've got some good choices! Do you know the exact GH and KH values? Also what size tank? Cardinal tetra are the ones that pop to mind right away, but they need GH and KH values under 4 which is why it would be helpful to know the exact numbers. Discus also thrive in this type of water but are very high maintenance. Rummynose tetra are another good option if it's a larger tank. They school really tightly and look stunning in groups bigger than 10. Mosquito rasbora will also do well if there is nothing in the tank to eat the little guys.