If the water you're using to fill the tank is from a municipal water system, it's chlorinated enough that there really isn't much potential for concern. If it's from a private well, you could add a little household bleach to protect against contamination during the filling process.
I use a veterinary syringe to measure out 1 mL of bleach per gallon of tank capacity when filling from a nonchlorinated source, particularly when hand carrying water in moller jugs.
If the tank is sanitary and the cap is left on, it will not become contaminated merely from the passage of time.
To learn to see below the surface, you must adjust your altitude