Umm acid isn't a PH buffer....buffers are cation elements in the water that take in the acid anions so the PH doesn't change. Thats why is takes so little if you use RO water...very little or no buffering capabilities.

If you want the strongest PH down acid use nitric acid....if you can get your hands on it. Make some from ammonium nitrate and H2SO4. Phosphoric acid is best for plants and its what is used in most PH downs if im not mistaken.

HCL is hydrogen and chlorine...if you want Cl in your plant go for it.