All pH minus products are some form of acid. Usually, pool supplies are just bigger containers. If it is a dry, granular product it likely lists sodium bisulfate as the active ingredient. If it is a liquid, it might be hydrocloric acid. Although someone posted a valid concern regarding the use of chemicals for a purpose other than what the label specifies, either will bring down your pH and Alkalinity. Just be sure you calculate the correct dose for the size of your tub.