Indeed Chlorine is highly unstable in water and gases off. But Chloramines(chlorine reacted with ammonia) do not. Check if your tap water has chloramines added by taking a look at your local water quality report for your town/city, or you can check yourself, Simply test your tap water with a standard ammonia test kit, if it registers anything other than 0 you have chloramines in your water.Well, you don't want to add standing water to your tank. You can remove chlorine through aeration. If you have chloramine, though, it needs to be removed chemically.
How do you know if you have chloramine?
How do you know if you have chloramine?