"All strong mineral acids are extremely dangerous. Those most commonly met for hydroponic use are phosphoric and nitric. Hydrochloric acid (muriatic, or ‘pool acid) is readily available, but should never be used in hydroponics because it adds unwanted chloride ions."
http://www.hydroponics.com.au/what-acid-should-i-use-for-ph-control/
phosphoric acid probably works like a fert, but might be the safest. it is possible to use new battery acid which is sulphuric acid. Use the ppm level to create your pH, meaning that if the ppm is dropping and the pH is rising, this most likely means that the plants are eating well. I'd say raise the ppm which will cause pH to drop and maybe don't worry so much about controlling pH with additional acids, and see how that works in the plant grow.