How to dechlorinate a pond with fish

Updated April 17, 2017

Keeping fish in ornamental ponds requires careful monitoring of the pond water chemistry to ensure the health and life of the inhabitants. Pond water is generally not replenished on an ongoing basis, such as from a stream in and out of the pond. The pond water can become a basin for the collection of chemical runoff and fish waste. Regular maintenance typically requires a 10 per cent water change on a weekly basis with 50 per cent water changes in the fall. Chlorine should be removed or neutralised from the new water prior to introduction to the pond.

Drain the pond water according to recommended guidelines for your pond size. Remove the water by siphoning with a garden hose or by using a pond pump to expel the water from the pond. If necessary, use several lengths of hose to ensure the drained water does not run back into the pond. General recommendations suggest 10 per cent weekly water replacement with a 50 per cent replacement in the fall. Topping off the water does not remove toxin build-up.

Move a large container, such as a clean trash bin, barrel or trough, near the pond. The container should be free of debris, chemicals or other potentially hazardous contents. If in doubt, use a new container. Move the garden hose near the container.

Fill the container with water. Fill it to a level that still allows you to dump, pour or drain the container into the pond.

Add the manufacturer recommended amount of dechlorinator for the volume of water you are treating. Dechlorinator treatment volumes differ by concentration and brand so follow manufacturer instructions. Stir the water in the container to allow the dechlorinator to neutralise the chlorine.

Pour the water into the pond. Repeat until the pond is full.

Test the pond water for chlorine content with a chlorine test kit prior to adding dechlorinator. Chlorine will naturally dissipate from the water over time.

Fill a 5-gallon bucket with water. Add dechlorinator in sufficient quantity, according to manufacturer instructions, to treat the whole pond for the chlorine levels detected.

Mix the water and dechlorinator in the bucket.

Pour the contents from the bucket evenly around the edges of the pond. If possible, use a paddle to stir the pond contents to allow the dechlorinator to mix evenly with the pond water.


If you do not know the volume of water the large container holds, fill a 5-gallon bucket with water and pour the water in the container. Repeat until the container is full, while counting the gallons of water required to fill the container. Generally, if the water supply is from a municipal water supply and if less than five per cent of the water is changed weekly, it is not necessary to dechlorinate the new water. Always test the chlorine levels to be certain the concentration is not harmful. If you are uncertain, dechlorinate even small volumes of water.


While fish may survive in less than pristine water, optimal health requires regular pond water maintenance. Many toxic chemicals and nitrates build up in the water column. Removal of the build-up is directly proportional to the amount of water changed. For example, to reduce 50 per cent of the nitrates, 50 per cent of the water must be replaced. The volume of dechlorinator used to treat the water is dependent on the type and concentration of the dechlorinator. Follow manufacturer instructions for optimal results. Irregular maintenance may result in toxin build-up that causes algae blooms, lack of oxygen, cloudy water and fish death. Treating symptoms, such as algae blooms, without treating the root cause, toxin build-up, may contribute to the decline of the health of your fish.

Things You'll Need

  • Garden hose
  • Pond pump
  • Large container
  • Dechlorinator
  • Chlorine test kit
  • 5-gallon bucket
  • Paddle
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About the Author

Skip Shelton has been writing since 2001, having authored and co-authored numerous articles for "Disclose Journal." He holds a Bachelor in Science in education and a Master of Business Administration with an emphasis in management from Northwest Nazarene University. Shelton also operates a small automotive maintenance and part-replacement shop.