I use Muriatic (Hydrochloric) Acid mixed with Hydrogen Peroxide, 1 part acid (28%) to 2 parts Hydrogen Peroxide (3%). From what I have read, if the solution turns brown, it needs regeneration which can be done with an air bubbler. The solution is cheap enough that you can just neutralize it with baking soda and throw it away. The acid is widely available at Home Depot or pool supply stores, but make sure it is Hydrochloric Acid; I have seen Sulfuric Acid labeled as pool acid.
Bob