Do not mix phosphoric acid into tank water. It creates so many issues. Just do an R/O and tap water blend to a KH target. In this case, I'd recommend a 50/50 or 75/25 mix of R/O to tap. Discus will thrive, spawn and readily hatch out fry at 180 ppm. I'd recommend you do less things, but do it consistently. Make R/O, mixes it up, and change water frequently. Do nothing else to play with water chemistry.
Good luck, Willie