I would slowly increase the kh using a mixture of tap water an RO water. I'm sure you know what kh is out of the tap (I did not see it posted), but my concern is with the high nitrates and low pH, especially since you're having to guess at both. You will be able to raise the pH as your buffering capacity increases. You can use crushed coral as was suggested, or Seachems alkaline powder or even baking soda will add acid neutralizing capacity to the water. I would probably go with a powder so that I could dose the partial water changes and control how much I raise the kh and how much anti-acid I add to the tank.
