Congo tetra, Phenacogrammus interruptus
Chlorine is added to all domestic water supplies in order to disinfect it. Unfortunately it is very toxic to fish and even more toxic to the nitrifying bacteria which we rely upon to maintain the water quality. chlorine is a very reactive gas and when dissolved in water part of it forms hypochlorous acid HOCl and it is this that does all the damage. Because Chlorine is so reactive and unstable it is sometimes mixed with ammonia to make chloramines this makes it far more stable and as a result the hypochlorous Acid is released at a slower and more concentrated rate.
Fortunately there are many proprietary aquarium water dechlorinator they have an active ingredient Sodium Thiosulphate which removes the chlorine within a matter of a couple of minutes. If chloramines is in the tap water make sure that your dechlorinator can deal with ammonia along with chlorine (some only remove chlorine) if one of these were to be used on chloramines there would be some ammonia left in the water and this would also be very harmful to the fish.
If you don't want to use chemicals to remove the Chlorine then there are a couple of alternatives, leave the water to stand for 24 hrs whilst using strong aeration, or filter the water through some new high grade activated Carbon. Both will remove all the chlorine. This will not work with water treated with chloramines.
In reality chlorine is not really likely to present much of a problem in the aquarium it is only added in very small amounts and this is enough to be effective when it is added. Most of the added chlorine is removed by organic material in the water so that when it reaches us only a small percentage is left active and this would be made inactive when it comes in to contact with aquarium water making it very unlikely to cause the symptoms mentioned. But is it worth the risk? Probably not.
Chloramine is completely different. Chloramine remains quite stable in water for several days and it is important that it is removed before the water is added to the tank.