WHAT are you using to read the pH?!?!? Perhaps your instrument is FLAWED?

As far as "pH down" you can use hydrochloric acid or sulfuric acid. HCl is not harmful to fish as the Chlorine it will leave behind volatilizes into the air. H2SO4 leaves behind nutrients for the plants. My local Lowes sells 2 gallons of 31% HCl for $9 or so. Compare that to your $20 8 ounce bottle of pH down, and you will appreciate the savings!