I know sometimes you're in a pinch and need to water your plants, but when it comes down to it, it's never a good idea to use tap water!
A lot of the time city water systems use chlorine to flush out harmful bacteria from our water systems to prevent citizens from contracting harmful viral bacteria, well this is great for us (kind of) it is not great for your soils microbial life, and in turn your plant's root system. The beneficial bacteria in your soil is key for nutrient uptake and prevention of root problems such as root rot, bad bacteria and fungi that can overpower your plant quickly! Another down side to tap water is it will have a high PPM which can throw off your fertilizer regimen and make some of your vital nutrients lock out!
It's always best to use more pure forms of water, such as Reverse osmosis (make sure to add extra Cal-Mag), distilled or even spring water, watch your PPMs in the latter. If you have to use tap water, allow it to sit out for at least 24 hrs with an air stone running before using, to allow the chlorine to dissipate, or add hydrogen peroxide; in which case you will still need to reintegrate the beneficial bacteria and fungi your next watering. Hydrogen peroxide works on chlorine in short by oxidizing its acid form which makes it fall apart into water and oxygen.
Here are a few GREAT Beneficial Bacteria Aids: