Originally Posted by Pepper2
Add 1/4 cup of bleach for each 15 gals. of tank capacity to holding tank. Make sure to fill water tank. Make sure water heater is off. Using the pump, run water thru all faucets till you smell bleach coming out. Leave in system for at least 3 hrs.. Drain and refill with fresh water. Run the fresh water thru all faucets. Drain tank. Repeat till you no longer smell bleach coming out of faucets.
I would add to this that you should fill the inlet to the top after running the faucets. The water heater will draw down the water level and the fill hose will have no solution in it. I have found things, that you don't want to think about, growing inside the fill pipe.
Of course use the same, or stronger, solution on the refill.