I use ascorbic acid, which of course is merely vitamin C. Very safe, I dunk my hands in the stuff on occasion when working on my acid reservoir and no problems whatsoever. But sulfuric acid, I would highly discourage anyone from ever using that for hydroponics. Wayyyyy to strong. I had to neutralize a half liter of it at my lab once and it took me something like 7 or 8 pounds of baking soda and when I tested the ph to check my progress, it just at the litmus paper! Maybe I'll start using vinegar after the current stuff is gone to save money, anyone else use vinegar?