The traditional way to coat steel water tanks is Portland
cement. It keeps the water fresh and protects the steel.
I use a 5% solution of Sodium hypochlorite (bleach) to kill any bacteria. 5% is the standard strength available in every supermarket. Do not confuse it with toilet cleaner. That stuff has thickening agents and other additives you don't want in your water tank. The stuff you want is pure bleach, thin as water (the cheapest stuff).
It is mixed 1 : 50,000 for normal use, 1 : 5.000 for shock treatment. Obviously the latter is flushed after 30 minutes.
Does it taste bad? Sure, it has a faint smell of bleach, just make your tea or coffee a bit stronger. In many parts
of the world this amount of bleach (and more) is standard in tap water. So, depending on where you're from, you may not even notice.