Chlorine is one of the main problems in tap water because it's harmful to fish at very low levels. Although some chlorine escapes when tap water is left to stand for several weeks, and some also escapes if water is added via a spray attachment, until recently the only sure way to remove all chlorine has been to add a water conditioner.

These conditioners bond with chlorine to make a neutral chemical compound that will not harm your fish. Always follow the manufacturer's instructions when adding any treatment to your pond, and make sure you know exactly how much water it holds.