In some countries monochloramine is used to disinfect water supplies rather than Chlorine. It is used because chlorine is a strong oxidizing agent and reacts with other compounds in the water supply to form nasties which monochloramine does not. It is used in about 25% of the USA.
It is a requirement of the Department of Health in NZ that chlorine is used as it is a better disinfectant than monochloramine. All proteins are a double helix of phosphates with various amino acids hanging off them and all living things contain proteins. Chlorine reacts with amino acids and other nitrogen compounds to form monochloramine, then dichloramine and then trichloramine. This is called the chlorine demand when chlorinating water supplies and there will be no free available chlorine until this demand is satisfied. As the chlorine dissapates the equilibrium changes and the chloramines move back to monochloramine and this is the compound that irritates your eyes in a swimming pool and the compound that people think they are smelling as chlorine. The way to fix that problem is to add more chlorine and push the chloramines up to the trichloramine state and this is not so irritating.
It is always the case therefore that chlorinated water supplies contain chloramines and they will not be removed by storage, heating or aeration, but will be moved to the monochloramine state which is the one that is most dangerous to your fish. The only way to remove the monochloramine is with chemicals such as sodium thiosulphate which is the active ingredient in treatments available from the pet shop. Removing the monochloramine this way will release some ammonia but it is generally in the more harmless ammonium state.