I use a 1-1-1 mix of 40% Peroxide (from the beauty supply store), Hydrochloric acid (from the pool supply store, Muriatic acid), and distilled water.
Remembering the old adage " Acid to water, just like you Otter", I assume you add the acid to the water, and
NOT the other way around, which can sometimes become "unstable"...
(Unless the peroxide somehow acts to stabilize the mixture....)