You can try a few things to naturally lower ph and soften water. Adding more wood without presoaking it will turn the tank water tea color, but also decaying woods will slowly soften water and lower ph. You can also try almond leaves or sphagum moss or peat moss in a nylon bag and add it to the filter. If you want to take the route of using chemicals, I have always had luck with black water expert, and seachem buffers. They make a discus buffer which should put your ph in a 5.5-6.8 ph range. They also have acid buffer which will drop ph but should best used in small doses and tested after adding to the tank.