KH and pH and everything in between

I have soft water. My tap water is soft, the water i use to do water changes is soft (RO water) and my fish love it. My KH is a 1dKH. BUT, I need my KH to be like... 3-4 dKH. That being said my pH is perfect, right where I need it to be. How do I raise my KH without spiking my pH? Any ideas?