Tap water is acceptable for topping up a low pond but it’s not ideal for filling the whole thing. However, most homeowners have no other options for keeping their ponds full. There’s no need to bring in purified water by the truck load at high cost when you can treat your tap water yourself to make it appropriate for fish and plants. First, most tap water is high in chlorine from treatment. Chlorine will evaporate out of the water if you simply let it stand in buckets for 24 to 48 hours before adding to the pond. This is great for a routine top up or water change.
For filling a pond the first time with tap water, simply plan not to add plants or fish for a few weeks as you treat and test the water. Many municipal water supplies are also high in nitrates, much like pond water months of fish feeding and waste accumulation. Nitrates are easily treated with water conditioners available from any pond supply store, but the liquids need time to act on the pond. Give your water a few weeks to adjust in pH as well, especially if you need to raise or lower the number. Tests for nitrates, chlorine, and pH are all available from pond supply stores. Within a week or two of warm weather, you should be able to adjust a pond filled with tap water into a safe environment for fish and plants.
