This is an excerpt from an article written by The Center for Agriculture, Food and the Environment:
"In our experience, the most effective and widely used acid is sulfuric acid; however, this is one of the most hazardous acids to use. For low amounts of alkalinity removal, phosphoric acid may be the acid of choice. However, we do not advocate adding more than 2.25 fluid ounces of this acid to 100 gallons of water, because of the amount of P one would add. Nitric acid is theoretically ideal because it adds nitrate nitrogen; but it fumes and is highly oxidizing, making it very difficult to handle. Citric acid is a weak organic acid and a solid, making it safer than the other three; but it is much less effective, and therefore more expensive to use."
Simply copied and pasted with no adulteration.
I do not appreciate anyone spreading misinformation on this site as I take a personal stock in the information provided here. I've noticed, over many years and several boards, there are people that logon to make a name for them self, show-off, to stroke their ego with delusions of grandeur. Some pull their information from magazines, others rely on their buddy's expertise or what ever sales pitch the local hydroponics store sales clerk swears by.
DO NOT BE FOOLED BY GIMMICKS!
Simply, the cheapest and easiest way to pH down is sulfuric acid, it was $8 last I bought some and I never used it all.