Chlorine and chloramines are harmful to fish and will kill the beneficial nitrifying bacteria in the pond. Likewise, they may burn or kill aquatic plants. Chlorine, a volatile gas, will dissipate with water circulation and exposure to the air within one or two days. Chloramines, however take much longer to break down. City water suppliers are more frequently adding ammonia to combine with chlorine to product the longer-lasting chloramines. It is not uncommon for city water to test positive for ammonia straight out of the tap.