First, for a healthy pond ecosystem, the water cannot contain chemicals that are harmful. Tap water contains many chemicals and minerals, most are added to make water clean and safe for humans. The same chemicals that make our water safe can make it deadly for fish and beneficial bacteria. So, to understand how to treat your pond water correctly, you first must understand your local water supply. Ask your water company for a chemical analysis so that you will know what chemicals are added to your local supply.
Chlorine is one chemical that is used to treat drinking water.
Chlorine can removed by filtration or letting the water stand in a bucket or barrel for a few days, allowing the chlorine to evaporate. As a result, many water districts are replacing chlorine with chloramine to disinfect their water supply. The only way to effectively filter out chloramine is by distillation or reverse osmosis, both of which are expensive and energy-consuming methods. Amquel, and products like it, also has the added benefit of dealing with excess ammonia in the water. There are other water treatments available for other possible chemicals.