HTH, 7 LB, PH Decreaser, Dry Acid Adjuster, Lowers PH In Pool, Alternative To Muriatic Acid. Active Ingredient-Sulfuric Acid and Monosodium Salt 91.5-94.7%. hth(TM) pH decreaser, 7 lbs., lowers the pH and total alkalinity of swimming pool water. The ideal pH range is 7.2-7.8. Maintaining pH in the proper range will allow your sanitizer to work more effectively, reduce equipment corrosion and reduce skin dryness.