Granulated stump remover is an excellent oxidizer when powdered out.
“Pool Shock” releases chlorine gas when wet. Walmart stocks it in season. A friend warned that when mixed with brake fluid stand back as nasty things happen faster than you can stand back.
Me? I buy it to shake (a 1/2 tsp or so) in the toilet cuz it the chlorine kills the creepy black slime that forms a ring. A package from WallyWorld lasts a long time.