Probably the best way to clean a tank is to blast it with extremely hot water. This will kill most bacteria.

An alternative, IMCO, is to wash it out with a solution of baking soda (base) and then with a solution of vinegar (acid.)

Do this a couple more times: base/acid flush and you'ld have a naturally cleaned tank with no damage to you or the environment.

Hot water probably is the most ethical.