I've been using phDown (phosphoric acid) for a few years, and have never had a problem. Well, except for a few months ago when I mixed 5 gallons worth in a gallon jug. Kinda pissed-off the ladies, but a quick flush and all was good.

Anyway, I have 7.9 to 8.0 ph wellwater and use 1/4 tsp:gallon, bringing my ph down to around 6.7-6.8.

A former troubleshooter in here (stinkyattic) did the math for me a couple of years ago, and at my dilution rate she thought I was fine. She's a chemist familiar with such chemo-numerical mumbo-jumbo.

The thread is around here somewhere...AH HAA...here it is, but don't let the title fool ya:
Humic Acid...