I’d like to add a note…it’s not really WHAT so much as it is HOW MUCH. 99% Hydrogen Peroxide is used to fuel the space shuttles. 50% is used in taxidermy and biological science(to take meat off the bone)and in uranium mining. 35% is used at the hair salon to bleach hair and is also used to treat drinking water for taste and odor and to treat wastewater for hydrogen sulfide.
All of the above can be very dangerous if misused (remember the guy in NY arrested for buying 35% Hydrogen Peroxide at a hair salon to make bombs?)
When you add a couple of bottles of 3% hydrogen peroxide to the water heater, you’re putting in 97% water…no big deal. It’s already diluted and it doesn’t take a strong concentration to oxidize bacteria and hydrogen sulfide. I mean, if the label says “mix with one part water and rinse your mouth to kill bacteria”, then how much damage would be expected to your equipment? (BTW, it does whiten teeth and gives them a squeaky-clean feeling and eliminates bad breath!):D