Where is H20 drain plug located, how to purify?

I use about a cup of bleach to 20-30 gallons of water in the tank. Run it through till you can smell bleach at the faucets. Let it sit a few hours or overnight. Drain, refill, reflush.
You are trying to clean the system, so you need a significant amount of bleach. If you just wanted to make the water in the tank safe to drink, then a few drops per gallon will work.