Lemon juice, vinegar or citric acid will bring ph down.
I wouldn’t worry so much with water temps i’ve grown mine with reservoir temps at a steady 85 degrees. When water starts going above 90 drop frozen gallon to get it back down between 80-85. If you go way down to 60 you’ll be shocking the plant. Last summer we had so many consecutive days in the 90’s I kept
nutrient temps at 85 didnt cause any damage to the plants at all.