ph down?

I did a little experiment with humic acid granules, dry grit that looks like course coffe grounds. I added a handful to one small bed (half barrel over 55 gal aquarium). I have several such systems that I do not manipulate the pH on, as they are breeding tanks and plant performance is of minor concern. My well water oddly seems to fluctuate between 7 and 8.2, much more commonly 8.2 than 7. And the AP tanks seem to chase the well water in pH, usually hovering in the high 7's. Anyway, the system with the added humic acid stays below 7, specifically not lower than 6.4 or higher than 6.9, and that was from one handful two months ago. Perhaps the humic acids buffer the AP to our desired range?

Cheapest acid? Muriatic (Hydrochloric), unless you live near a golf course and you can sneak over and steal battery acid from the carts :)