The Safe Drinking Water Act (SDWA) was enacted in 1974 to ensure safe drinking water in our homes in the United States. As a result of this law, the Environmental Protection Agency set drinking water quality standards for all public water systems across the country. These regulations require that all municipal water supplies be treated to limit bacterial growth and other hazardous contaminants.
Water treatment companies can choose different disinfectant methods and for a long time, chlorine was the disinfectant of choice; however, many municipal water treatment plants have switched to chloramine. For most people, it doesn't make any difference, but to the aquarium owner, the disinfectant in the tap water is very significant.
Chlorine is a gas that is dissolved into the tap water to kill microorganisms that may inadvertently enter the city water supply piping. The concentration of chlorine required to successfully treat public water sources is high enough to be lethal to your fish. Fortunately, it is easily neutralized by one of two methods. The first option is to chemically treat the water with sodium thiosulfate. Virtually every water treatment product available at your local pet shop contains this chemical. In other words, if your water contains only chlorine, all you need to purchase is an inexpensive water treatment product to make your tap water safe for your fish.
The second chlorine removal method is based on the fact that chlorine dissipates quite rapidly into the atmosphere when the water is exposed to air. Expose water to the open air for 24 hours, and it will become chlorine-free. You can accomplish this by leaving it in open buckets or by filling the tank and letting the filter run at least one day before adding the fish. Many old-timers in the hobby remember treating aquarium water in this way. It worked quite well because at the time chloramines weren't used in public water treatment. Aerating the water (with an airstone attached to an aquarium air pump) will allow the chlorine gas to dissipate even more quickly.
Sounds fairly simple? Not so fast. Since many municipal water plants have switched to chloramine, treating tap water for aquarium use has become a bit more complicated.
Chloramine is a combination of ammonia with chlorine. The ammonia binds the chlorine gas to keep it in solution longer. Unlike straight chlorine, which dissipates fairly quickly when exposed to air, chloramine remains in the water longer. That's good for the water 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 chloramine will still be there. Secondly, it means you must ensure that you treat the water for ammonia as well as for chlorine. Not all aquarium water treatment products will neutralize the chloramine. Even those that claim they do aren't always fully effective in 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 test may not be accurate, it may still indicate that the ammonia is present in the water, even though it is in a bound, non-toxic form. 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).
Do You Have Chlorine or Chloramine in Your Water?
The most direct way to determine what is in your tap water is to call your water company and ask what they use to treat the municipal water supply. By law, they must make the composition of your water available to you. Be aware that the city water supply company may change water sources and chemical additives at a different time of the year, depending on water temperature and rainfall amounts, or other factors.
You can always test your tap water yourself. It's wise to test your water anyway, so it's a good route to go. Test kits available that look for chlorine as well as chloramine. Or, you simply test your tap water for ammonia. If it tests positive for ammonia, chloramine is almost certainly present. Then you can choose the proper product to treat your tap water.
Another option is to bypass all testing and simply treat the tap water with a product that neutralizes both chlorine and ammonia so you cover all the bases. Regardless of what you do, it's wise to be aware of what's in your water source to ensure it's safe for your aquarium and pond fish.
National Primary Drinking Water Regulations. United States Environmental Protection Agency.
I Use Municipal Tap Water in My Aquarium. What Do I Need to Know? Duke University.