the chances of it getting infected with plain water are very low. depends on what sanitizer you use, but basically none of them will break down in the time it takes for your wort to ferment and for you to refill it.
i’ve been brewing for a long time, never had anything get infected, and i always use diluted phosphoric acid (starsan).
think about it this way; would you rather have sanitizer (safe, food grade), or soap (possibly both emetic, and laxative) in your wort?