Chloramines are a combination of ammonia with chlorine. Unlike straight chlorine which dissipates fairly quickly when exposed to air, chloramines remain in the water. That’s good for the company tasked with keeping public drinking water safe from contaminants such as bacteria. It’s not so good for those of us who keep fish and want to use our tap water to fill the aquarium.
First and foremost, it means the old trick of aging water in open buckets or in a tank with a filter running, won’t work anymore. You can age the water for days and the chloramines will still be there. Secondly, it means you must ensure that you treat the water for chloramines rather than for just chlorine. Not all aquarium water treatment products will neutralize the chloramines. Even those that claim they do, aren’t always fully effective getting the job done, so choose your water treatment products carefully.
Often these products remove the chlorine portion and lock the ammonia portion, in the same fashion as Ammo-Lock. This will render the water safe for your fish, but keep in mind that your ammonia tests may not be accurate. If you want to accurately monitor your ammonia levels, you’ll need a test kit that can separately measure NH3 (free ammonia) and NH4+ (ionized ammonia).