I always tell customers to use CLR, which is safe to use.
A more harsher product is Muriatic Acid, but it is harmful and corrosive if used in closed areas. Need plenty of ventilation when using this product.
Those disks that go in the tank, along with the cartridges that leak cleanser in the tank are notorious for eroding away the rubber components inside of a tank, and is not recommended.
“Your best interest is secured by making the right decisions the first time.”