How Long Does Water Need To Sit to Dechlorinate?

How Long Does Water Need To Sit to Dechlorinate?

It will take about five days for about 10 gallons of tap water with normal levels of chlorine (about 2 ppm) to dechlorinate. 

Now that you have the answer to the question of how long does water need to sit to dechlorinate, let’s talk about it for a bit. So where does chlorine in tap water come from and why they need to dechlorinate water before adding it to our aquariums? 

Well, health protection agencies around the world require water utility companies to add chlorine or chloramine to water to kill bacteria. Chlorine is normally the chemical added to water to kill bacteria because it is more effective.

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While chlorine is very effective in killing bacteria, it is pretty much harmless to human beings. So the water that you draw from the tap pretty much has some chlorine in it. You may not know it because you have been drinking it all your life but it is there and it will not harm you.

While Chlorine is pretty much harmless to human beings, it is toxic to fish. In other words, if you just draw water directly from your tap and add it to your aquarium, you will be poisoning your fish. If you are a new fish parent, you may have just recently learned this the hard way. Don’t beat yourself up over it. To err is human. 

So because the chlorine in tap water is toxic to fish, there is a need to dechlorinate tap water before adding it to a fish tank. The good news is that you actually do not need to do anything to remove chlorine from tap water. This is because the chemical is pretty volatile and will fully evaporate from water if it is left to sit. 

Generally, 1 ppm of chlorine in water will take about 55 hours to evaporate. The concentration of chlorine in tap water is normally about 2 ppm (parts per million). So if you draw about 10 gallons of water and let it sit, it will take about five days for it to fully dechlorinate. 

In other words, it will take about five days for all the free chlorine in it to evaporate. If you think this is quite a long time to wait then you are not alone. It is because of this reason that several methods have been devised to speed up the dechlorination process. Some of the acceptable ways of speeding up the dechlorination process include aeration, circulation, and the use of UV (ultraviolet) light. 

Boiling tap water is also an acceptable way of speeding up the evaporation of chlorine from tap water. 

Methods that can be used to dechlorinate Water

So, as mentioned above, letting water sit is the way most people primarily make their tap water safe for aquariums. Nevertheless, there are several other chemical and chemical-free dechlorination methods that can be used to make tap water safe for aquariums. 

The primary method of dechlorinating tap water using chemicals entails the use of water conditioners. Water conditioners are special chemicals that can rapidly and efficiently remove chlorine from tap water.

There are many water conditioners on sale online. However, while the use of water conditioners is effective and saves time, the conditioners themselves are made of certain substances that are not very healthy for fish. This is why many people prefer chemical-free methods of dechlorinating tap water e.g. letting water sit. 

However, as I have highlighted above, letting water sit to fully dechlorinate takes a lot of time and nobody likes waiting. Below are some of the other chemical-free ways of dechlorinating water. I mentioned some of them earlier. 


Aeration, as a method of dechlorination, essentially just speeds up how fast free chlorine leaves the water. 

Generally, people who use aeration as a method of dechlorination draw tap water into a container and use an aerating sprayer or a spraying nozzle to add air to it. As the sprayed air escapes from the water, it escapes with chlorine since the chemical is highly volatile. 

Circulation of tap water using is also thought to work in kind of the same way as aeration to remove chlorine from aquarium water. 

Use of UV light 

The use of ultraviolet light is considered to be one of the natural ways of making tap water safe for aquariums. This method requires you to buy a UV sterilizer. The good news is that UV sterilizers are generally not very expensive. Many high-quality UV sterilizers go for just $150. 

UV sterilizers dechlorinate tap water by bombarding it with ultraviolet rays. When set correctly, the sterilizers can remove both chlorine and chloramine from tap water. They also have the additional benefit of treating tap water and making it extremely safe for fish

As I have mentioned above, UV sterilizers need to be set correctly to remove chlorine and chloramine from water. Therefore, when buying a UV sterilizer, make sure it can bombard water at the 254 nm wavelength and with a density (radiant) of 600 ml. Because this is the kind of UV light that can remove both chemicals. 


Boiling tap water is one of the fastest ways to remove free chlorine from it. The method simply involves drawing tap water and boiling it. When you boil tap water, the heat plus the aeration and the circulation from the boiling will get rid of chlorine in it. Make sure you boil tap water for at least twenty minutes to remove all the chlorine from it. 

As I have mentioned, the biggest benefit of this method is that it is very fast. Another benefit is that it also removes chloramine, a chemical that is also used to treat water and that is harmful to fish. 

When you use this method to dechlorinate water, make sure you wait for the water to cool before adding it to your aquarium. If you don’t, you will end up turning your pet fish into a stew. 

Adding Vitamin C

Adding one teaspoon of Vitamin C (ascorbic acid) to 1 gallon of tap water quickly neutralizes all the chlorine and chloramine in it. The use of Vitamin C to dechlorinate water is also considered to be a natural way to make tap water safe for aquariums although it obviously involves the use of a chemical. This is because the chemical is one that is beneficial to fish and that fish need for good health and wellbeing. 

Alternative sources of water for aquariums

Apart from tap water, you can also use alternative sources of water for your aquarium. They include well water, rainwater, distilled water, and filtered water.

You can use well water as aquarium water but you need to have it tested to make sure it is safe for your fish. 

Rainwater is generally safe but you have to store lots of it to use even when it is not raining. 

Distilled water is distilled water and you can buy it cheaply at your local convenient store. However, it may not be a practical source of water for your aquarium if it is big in size.

Filtered water that is filtered by either deionization or reverse osmosis or both. If you have installed a filtration system in your house, you can add the filtered water directly to your aquarium. It is very safe. 

How to test tap water for chlorine levels

As a fish keeper, you have no option but to learn how to test tap water for chlorine levels. This will help you to know how long to wait for it to dechlorinate if you want to let it sit. It will also help you to confirm if you have removed it successfully after using any method of dechlorination. 

The easiest way to test tap water for chlorine levels is to use chlorine level test strips. You can buy like 200 test strips for less than ten bucks. To test chlorine level using a test strip, you simply need to dip one into the water and remove it quickly to compare to a supplied color chart.

When buying chlorine level test strips, make sure you buy those that measure really low levels of chlorine (0 to 4 ppm).