Will the tap water I keep in my fridge ever get “old” and dangerous to drink?

Most tap water is treated with chlorine and, even if some microbes live through that treatment, there is nothing in water for them to live on. (Pathogens need an energy source in order to grow.)
 
However, if you use up only part of the water in your pitcher, it’s still a good idea to empty out the rest before refilling the pitcher. After awhile, water develops a stale, flat taste.
 
Source(s):
Susan Brewer, Ph.D, University of Illinois, Department of Food Safety and Human Nutrition