We use food grade hydrogen peroxide (35%). The biggest thing though, in my my opinion, is using the sun and letting it dry..
I dump our tanks, scrub them a bit, then pour some food grade hydrogen peroxide on them and slosh the tank around so it gets on all the surfaces. Then I let it sit for 5-10 minutes and then rinse it and scrub it the rest of the way. After that it gets left to dry completely in the sun. I just fill some tubs with water and leave those for the horses.
After its all dry, I put it back and fill it, adding 1/8 cup of the food grade hydrogen peroxide in my 100 gallon stock tank.
You should only use food grade hydrogen peroxide (35%). It is the only type of hydroen peroxide that is meant for consumption. Be careful not to get the undiluted stuff on your skin. It burns and itches.