Question: how to add hydrogen peroxide to water heater?
Answer: Add 1 to 2 pints of 3% hydrogen peroxide (the kind typically found at drugstores) for every 40 gallons of water heater tank capacity. This can be done by pouring the peroxide into the water heater's outlet pipe opening or into the T&P valve opening; alternatively, you can use the pre-filter screen method (seen here).