This used to be a simple question with a simple answer. Nowadays,the answer is less straightforward, but fortunately still manageable. The major safety issue for fish with tap water is chlorine, which is added as a disinfectant to keep harmful bacteria and other germs from making people sick. While it’s kept at a safe level for humans, it’s essentially a corrosive that can burn and damage sensitive tissue like gills. It’s nasty stuff to fish and it falls to us to protect them. The traditional method for removing chlorine from tap water is simply to let it sit for a day or two so the chlorine can evaporate naturally. However, water utilities don’t find chlorine’s easy evaporation to be ideal for their purposes, and some have begun to use a new additive: chloramine. Chloramine is a combination of chlorine and ammonia, and is deadly to both fish and the beneficial bacteria colonies that support your pond. Even worse, the stability that makes it cost-effective to the water utility also makes it much harder to remove for the sake of your fish. While chloramine theoretically can evaporate over the course of many days, in practice it requires chemical additives to ensure complete removal. To complicate matters, different utilities use different chemicals and combinations, so you’ll need to test your water or ask your utility company to see exactly what they use.
Once you know what you’re dealing with, the easiest and fastest way to remove both chlorine and chloramine from water is to use a dechlorinator product. However, a product that simply treats the chlorine will leave behind ammonia, which is still another deadly poison for fish! A better option would be to use a water conditioner that treats both chlorine and chloramine. This method has the advantage of being instantaneous, so that water can be added to your pond immediately after the treatment is added. Another option is to use an activated carbon filter in your pond. Normal biological and mechanical filters can’t effectively manage chloramine in tap water, but activated carbon can. If you already have a filter box for your pond you can simply add some to the filter media. As water flows through the filter, chlorine is removed through adsorption, which sidesteps the ammonia problem altogether.
