Dry acid is used to lower pH and Alkalinity.
To low pH you would need to disperse small amounts across the water.
To lower Alkalinity the process is a bit different. You would add anywhere from 5oz - 15oz into the deepest area of the tub in a concentrated area to create a 'Bubble Effect' causing the Total Alkalinity to drop and only temporarily affecting the pH.
Active Ingredient.............. Sodium Bisulfate 94.5%
Available in 20oz, 2lb, or 6lb