Chlorine is a strong oxidizing agent which reacts with nitrogen compounds in the water to form monochloramine which reacts with more chlorine to form dichloramine which reacts with more chlorine to form trichloramine. When this happens it is then possible to have "free available chlorine." As the chlorine disperses or is used up by other contaminants (like people urinating in a swimming pool, fish in an aquarium or aeration to remove chlorine) the equation moves back towards the monochloramine. The creation of chloramines (and other chlorine compounds) is called the chlorine demand. When you swim in a chlorinated pool and your eyes sting it is not too much chlorine, it is not enough and the equation has moved back towards the monochloramine. Any water supply which is chlorinated will contain chloramines. Monochloramine is added to water supplies in the United states as a disinfectant rather than chlorine to avoid the creation of some of the other chlorine compounds. It is a disinfectant (not as good as chlorine) and as such cannot be doing your fish any good. If you wish your fish to swim in disinfectant that is your choice but if monochloramine makes your eyes sore in a swimming pool I would suggest that your fish would prefer not to be swimming in it. In my view any chlorinated water should be treated to remove chlorine and chloramines.