Chlorine is added to all city water and can be very toxic to koi. It can burn their gills and can cause death. Fortunately, chlorine evaporates rather quickly. Adding small amounts of city water through an auto-fill, or a very slow-flowing drip line usually does not present a problem.
But if you replace more than 5%-10% of the total volume in your pond, you’ll need to take steps to remove the chlorine. The most common way to solve this problem is to add dechlorinator (sodium thiosulfate) to the pond before you add the fill water. Another solution is to install a carbon filter on the fill water line. A good quality carbon filter will automatically remove all chlorine from the water before it reaches the pond. This is a great solution, but you need to make sure you change the carbon in the filter regularly.