Chloramine (generally monochloramine NH₂Cl) is a combination of chlorine and ammonia used in many municipal tap water suppliers to remove bacteria from the water. Chloramines are less prone to dissipation by long runs of piping and is considered less of a carcinogen than chlorine.
Chloramine does not kill bacteria firmly ensconced in a biofilm on the pipes. If a municipality uses chloramine, every once and a while it must pulse a high concentration of pure chlorine into the water to kill the bacteria in the biofilm. If you happen to be doing a water change when that “pulsing” is done, you can kill a lot of fish.
When a system is on chloramine. the chlorine pulses need to be frequent. Depending on the water quality it might be as often as weekly. This is why some suppliers of conditioners recommend five times the level of conditioner for chloramines that is used for chlorine.
About one third of cities in the US use chloramines. To determine if you have chloramine, you can’t really test for it. If you neutralize the chlorine in chloramine with conditioner, then test for ammonia, you won’t get a good answer. The conditioner will interfere with the chemistry of the test. So, you need to call your water supplier and find out what they are using.
Testing for Chloramines
There is a problem that has occasionally killed tanks of fish. There are two types of chlorine tests. One type of test measures both chlorine and chloramine and is called a “total chlorine” test. One type of test ONLY measures chlorine, it doesn’t measure chloramine. It is important to make sure the test says “Total Chlorine Test”.
A few poor souls used the only chlorine test. They have then stopped using conditioner because “their test kits show they have no chlorine”. And these unfortunate souls have killed tanks of fish because they have chloramine in their water.
There are some aquarium hobbyists who are interested in delving deep into the science and the calculations behind all aspects of the hobby. For those who are so inclined the following is pertinent: