I dont mean to bump an old thread but i am reading through the archives and I figured I would update for others who do the same.

The liquid version of "General Hydroponics PH down" is food grade phosphoric acid and can be acquired at your local hydroponics store for about $30 per gallon. However, I havent been able to find the concentration percent of the product.

(My expanded shale had a little something in it that is PH buffering my system at 7.4 so i use this product to bring down my PH a little. I mix it in the sump which goes through the grow beds before getting to the fish tank, the fish do not seem to mind. Mozambique tilapia, bluegill, redear sunfish and channel catfish.)

brian