Theoretically, you should be able to use any acid to bring down Ph. DO NOT USE JUST ANY ACID, as this can be detrimental to your health. Hydrochloric acid is the best, and is available at most pool/spa shops
To reuce the pH in your pond you must use some sulphuric acid. Sulphuric acid will neutralise the pH of water thus reducing the pH concentration. Molecules from Sulphuric acid will make double bonds with water molecules easily dissociating the water molecules. This is the result how the pH decreases rapidly.
