For a tank your size...
Use 1 cup of bleach in about a gallon of water. Suck that all into the water tank, or pour it in someway. Then fill the tank with regular water, no more bleach.
Take the bypass off the water heater so it will fill. Then open all the faucets, with both hot and cold water, so you smell bleach coming out the hot and bleach coming out the hold. That means the water heater has been filled.
Let it all set for 12 hours or longer.
Drain the tank. Refill with fresh water. Open all the hot and cold faucets and run water until the bleach smell disipates. Drain the tank, refill, and run water until the bleach smell is completely gone.
Don't overdo the bleach. Enough is enough and more doesn't work better.
Don't forget the outside shower.
Mix a tablespoon of bleach in a gallon of water and pour it in your drinking water hose. Fill the hose as full as possible and screw the ends together. Let that set for a while and flush it out thoroughly. Slime can grow in there, too.
