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How to Dechlorinate Tap-Water for Your Garden Pond

If you are filling up your Garden Pond for the first time or doing regular water changes the chances are you are using water from the tap. If you have koi carp, goldfish, trout or any other type of fish or aquatic life in your garden pond then you should ensure that you dechlorinate it first. Your local water company will likely add either chlorine or chloramine to your water. This is done to ensure the water is clean for human consumption and while its fine for us it can be damaging to your filtration bacteria colonies as well as your pond fish.

The most commonly used chemical is Chlorine which is added in gas form to the tap water but depending on where you live and what policies your local water companies have in place, they may use chloramine instead. Chloramine is not a gas and therefore lasts longer in the water which helps to keep it cleaner for longer periods of time, while this is great for keeping our drinking water clean it can make removing it more difficult depending on the de-chlorination method used.

New Pond Syndrome

A brand-new garden pond and filter won’t have any of the beneficial bacteria required to convert the waste your pond fish produce from the highly toxic Ammonia to less toxic Nitrite, and then to convert the Nitrite to the much less toxic Nitrate. Most fish keepers will add beneficial bacteria to the biological filtration media in order to give the filtration a jump start. These beneficial bacteria can be killed off by adding unchlorinated tap water to your pond system, which will prevent the Nitrogen break down cycle from starting, leaving you with a bad case of New Pond Syndrome.

New Pond Syndrome does not only occur in brand new ponds or pond filters, but it can also occur in a fully matured pond system if you are not careful. While it’s important to keep your filter clean, if you wash the established pond filter media with water straight from the tap the chlorine will likely kill the bacteria colonies you’ve spent so long maturing, taking you back to square one with your filtration capacity and maturity. If you then continue to feed your fish as you did before when you had fully matured filtration media your water quality will dramatically reduce leading to an ammonia spike which can kill or harm your pond inhabitants.

Methods for Dechlorinating Your Pond Water

There are multiple ways to ensure you have clean chlorine free water in your garden pond, we will discuss the most common methods below, starting with our preferred and recommended approach.

Inline De-chlorinators

Our preferred method is to effectively remove all Chlorine & Chloramine, as well as any other contaminants that may be present at the source before they enter the pond system. The process is simple and highly effective, you simply connect the inline de-chlorinator in-between your tap and garden hose that fills your pond. The Tap water enters the de-chlorinator at one end and travels through the activated carbon, which absorbs and removes the chlorine and chloramine as it goes.

You must pay attention to the maximum flow rate for whichever de-chlorinators you purchase and ensure that you don’t exceed this while filling up your pond. The slower the flow the longer the tap water is in contact with the activated carbon inside the inline de-chlorinator, which gives it more time to absorb and retain the chlorine and chloramine before it enters your garden pond.

Obviously, there is a maximum amount of lifetime absorption for any particular de-chlorinator which means it will have a limited lifespan, usually expressed in total litres or gallons of water passed through. It is a good idea to fit a water meter to the hose so you can monitor how much water has been filtered with your de-chlorinator, so you know when to replace with a new one. Or if you are doing weekly water changes at known amounts you can calculate how many water changes can be completed before it needs replacing.

Liquid De-chlorinators

If you do not want to buy a dedicated inline de-chlorinator then the next best solution is to use a liquid de-chlorinator that you add to your pond. It’s the most commonly used method for fish kept in tanks but is also commonly used for garden ponds as well, just in much higher quantities.

You should add the liquid de-chlorinator when you initially fill the pond as well as each time you top up and do water changes. You must know approximately how much new tap water needs dechlorinating so you can ensure you use the correct amount of liquid de-chlorinator. Many of the liquid de-chlorinators available to fish keepers will also remove chloramine as well as the chlorine so please ensure whichever brand of liquid de-chlorinator you purchase that its rated to remove both.

Let’s say your garden pond and filtration is 10,000 litres and you fill it with tap water from your hose. Once filled you simply add the appropriate amount of liquid de-chlorinator as per the manufacturer instructions for 10,000 litres. Simply tip it into your pond, spreading it out as much as possible while you do so. You could use a watering can mixed with pond water and mix it throughout the pond to spread and mix effectively, or you can simply pour it in and stir with a stick or pole.

It’s a little more difficult to know exactly how much water you are putting in when topping up and performing water changes so it’s a good idea to have a visual mark and empty to that point each time so you know how much new tap water is entering the system. Lets say this point is for a 10% water change, or 1000 litres in our example above. You simply calculate how much liquid de-chlorinator is required for your 1000 litre water change and add it to the pond before you start filling but after you’ve emptied any old water that’s being replaced.

You can’t overdose liquid de-chlorinator so if you are unsure how much water you are putting in you are better off overestimating rather than underestimating.

Gassing off (Evaporation)

If you know for sure that your local water company uses chlorine and not chloramine then you can let the Chlorine evaporate before it enters your pond system. It typically takes 24-48 hours but must be left in an open top container and it must be big enough to hold the required volume of water needed for the task at hand. Adding an air stone to circulate the water will speed up the process. Its recommended to test the water for chlorine before you put into the pond as you won’t know if it’s evaporated fully. The longer you leave it the more chance all the chlorine will be gone.

As previously discussed, some local water companies use chloramine rather than chlorine to treat the water. As chloramine isn’t a gas it won’t evaporate in the same way chlorine does. That is by design to help keep your drinking water cleaner for longer without the risk of gassing off reducing its efficacy. This method is therefor not suitable for water with chloramine present.


Whilst rainwater is certainly free of chlorine and chloramine it may still contain other contaminants, particularly if you live in or near a city or industrialised area or factories. It’s not guaranteed to be 100% clean even if you live in a rural area, particularly if it has not rained for a long time. Its recommended that you filter any additional rainwater though an inline carbon filter before its pumped into your pond.

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