The idea isn't necessarily that microorganisms could mutate to become "resistant" in the same sense that they could to antibiotics - although given sufficient timescales I could actually see that being a concern. My main concern is instead that there are several common microorganisms which can survive the pH at which anionic sanitizers are typically used (2-3, roughly). As erock pointed out, there is also a surfactant component acting to "bind up" the little guys, but without a thorough rinse I don't feel comfortable relying on that to totally rid the tank of contaminants. I fully acknowledge that this is probably needless worrying.