The use of Chlorine to treat water on an industrial scale dates back to the 1890s in Germany and the UK. Today, Chloramine is often added with Chlorine as it is more stable. While a few countries in Europe have moved to other methods, the vast majority of urban piped water systems across the world continues to use Chlorine and Chloramine to eliminate harmful pathogens.
Today most urban tap water contains between 0.2ppm (parts per million) and 2ppm of Chlorine and Chloramines. This is high enough to kill water-borne diseases, but low enough for human consumption. However this level is sufficient to harm fish and destroy the beneficial bacteria in an aquarium. For sensitive fish species, levels as low as 0.05ppm can be lethal.
A snapshot the general published levels of Chlorine / Chloramine in some cities:
Tokyo: 0.1 to 1ppm
Chlorine dissipates by itself over time. A glass of water left standing will off gas most of its chlorine content in between 24 to 48 hours. So if your building receives water from a main water holding tank that has water left standing for a long time, it is possible for your tap water to have very low levels of chlorine by the time you turn it on, even if a high amount was injected during water treatment. Conversely, in residential households without holding tanks, where the pipes are constantly flowing, chlorine levels tend to be higher.
In Singapore for example, tap water usually has a chlorine level of around 2.5ppm. However, in office buildings where the water tank is large, the tap water that comes out can have near zero chlorine present due to the standing water off-gassing (say over a quiet weekend). But during weekdays, the chlorine levels rise due to the water tank being more constantly cleared.
In other words, chlorine levels can often vary.
Measuring zero today does not mean zero tomorrow.
In areas where the level of chlorination is low, it is possible to do smaller water changes without dechlorinators without killing fish. It may still cause cumulative damage in the long run however.
In some cities, such as Tokyo, the targeted chlorine levels is low (around 1ppm) and tap comes out for most folks to have between 0.1 to 1ppm of chlorine. In such places, hobbyists find that they can do 20% water changes (without dechlorinator) without livestock being affected.
However, this is a dangerous practice.
If the water chlorine level spikes ( if the pipes are cleared faster than usual for that day) or if they do a larger water change and hit lethal levels, they lose livestock.
It is generally safer to use a dechlorinator if you reside in a country where water chlorination is used, as tap water chlorine levels may change over time.