To make the pH more acidic, you should use either Citric Acid, Sulfuric Acid, or Acetic Acid. These should be very dilute. Do not use Nitric Acid or Hydrochloric Acid. If you wish to use Ascorbic Acid, that adds a twist to the situation as it is an antioxidant and a developing agent. It can be used, but has some side effects.