Fish

Even low levels of chlorine and chloramines can be extremely toxic to fish. Chlorine is a very powerful oxidizing agent and it is toxic to fish at concentrations of less than 0.05 mg/L. Water used for fish culture should not contain any residual chlorine to be considered safe. Chlorine and chloramines cause gill damage which eventually results in gill lesions. These lesions will eventually thicken the gill filaments leaving the fish unable to utilize oxygen and release carbon dioxide, thus resulting in fish death. Many of the 'Chlorine removers' (in liquid-form) from your aquatic store  will work to break the chlorine and chloramine bonds (chlorine + ammonia = chloramine) and reduce the chlorine and ammonia down to non-toxic levels.

Many of these chlorine removers contain sodium thiosulfate, which is widely used to break down chlorine and chloramines and is considered non-toxic to fish. Sodium thiosulfate was chosen because it provides a very effective solution for use in water gardens and ponds because of its ability to break the chlorine bond and chemically remove the chlorine after a short working time. Aeration of the water (which the majority of water gardens have present) will allow the ammonia, which was released from the chlorine bond, to escape as a gas over a period of one to two hours. Any remaining ammonia will be filtered out through flow through filters which most often contain activated carbon or some type of filter material.

Best used for regular water changes and when filling a new pond. Manufacturer recommends not putting in any fish or plant life until a day after treatment to be safe. This should give any residual ammonia enough time to escape or filter out.