How Long Should Water Sit Before Adding to Your Aquarium

By Eddie Waithaka @aquariawise

How Long Should Water Sit Before Adding to A Fish Tank

Nothing is worse than adding fish to your fish tank and having them die in less than a week, but this will certainly happen if the water you use is not safe for tropical fish.


To ensure the water you add to your aquarium is of the best quality, let it sit for at least 24 hours before adding it to your tank to remove the free chlorine used to purify tap water. If the chlorine concentration is too high, you may have to wait up to 5 days. This will allow all the chlorine in the water to evaporate. It will also give your filter time to cycle and remove ammonia and nitrite from a new fish tank before you add your fish.

You also want to let your water sit for 15 to 20 minutes after adding conditioner before you place it in your fish tank as well…

…but we’ll look at that later.

At this moment…

…let’s delve a little more into evaporating chlorine from aquarium water.

….and much more

How Long Should Water Sit to Remove Chlorine (for Fish Tank)

As mentioned above,

Let your water sit for 24 hours to 6 days to remove chlorine before adding it to your fish tank. Leave it exposed to allow ultraviolet rays from the sun to evaporate the chlorine. Aerating the water for 12 to 24 hours with airstones and heating (boiling) it for 30 minutes (or so) will also speed up evaporation.

I also recommend placing your water in a large container when looking to evaporate chlorine faster to allow more room for air and sunlight to penetrate.

That said…

How long it will take for the chlorine to dissipate may vary depending on the amount of water and chlorine added by your municipal supplier. A chlorine concentration of 2ppm (parts per million or milligrams per liter) in 10 gallons can take 4 days to dissipate. A lower concentration will take a shorter time.

Worthy to note as well…

Some water treatment plants do not use chlorine but chloramine to treat tap water, which is more stable and not as-easy-to evaporate. To remove chloramines, you will need to use a de-chlorinator.

Exposing water to the sun might be time-consuming, but it is the best way to remove chlorine in large amounts of fish tank water. You can try heating it to accelerate the process if you do not want to wait it out.

What is The Fastest Way to Remove Chlorine from Aquarium Water

Letting water sit in the sun is the easiest way to remove chlorine from water, but the fastest way is by using a water filter. You can boil and aerate your water to hasten the evaporation, but it’s still slower than placing a water filter on your faucet to remove the chlorine.

It can take up to 5 days to evaporate chlorine from tap water, depending on the amount added by your local supplier, but a plug-in activated carbon filter, like this Brit system, will remove the chlorine at the point of use.

You can also use other filtration systems, such as RO and AC filters, if you have them, though each has its pros and cons.

How to Remove Chlorine from Water for Aquarium

To remove chlorine from water for use in your aquarium, you can either:

1. Let the water sit in a sunny, well-aerated area inside a wide container to evaporate the chlorine.

2. Filter the chlorine from the water using an RO filter.

3. Remove the chlorine using an activated carbon faucet filter.

Now, as stated…

You can use a filter to remove chlorine from water for your aquarium or let the water sit in the sun for 24 hours (to 6 days) for the chlorine to evaporate.

Letting the water is easy and cheap but time-consuming, and using a filter is instant but expensive. You will need to purchase and install a filter before you use it.


Filters that can remove chlorine and chloramines from water for aquariums work differently depending on the filtering mechanism, each with pros and cons.

See the benefits (and the not-very-so) of using these techniques to remove chlorine from water.

Chlorine Removal MethodProsCons
Evapoation in the sun
  • Fairly easy to do
  • Quite cheap
  • Natural and eco friendly
  • Effective
  • Does not reduce water hardness
  • Time consuming
  • Cumbersome
  • Not accesible at point of use
Chlorine Removal MethodProsCons
Activated Carbon faucet Filter
  • Fast
  • Low tech
  • Removes chlorine at the point of use
  • Remove chlorine, chloramine
  • Does not reduce water hardness
  • Has an initial cost
  • You may need different cartridges
Chlorine Removal MethodProsCons
Reverse Osmosis Filter
  • Removes chlorine, chloramine
  • Removes metals
  • Removes chlorine at the point of use
  • softens the water
  • Reduces water hardness
  • Has initial and operating cost
  • High tech

How Long Does it Take for Chlorine to Naturally Evaporate from Water

It should take 24 hours at room temperature for chlorine to evaporate from water if the amount in your tap is not too high. If the concentration exceeds 2ppm (parts per million) of water, it could take up to 5 days to dissipate the chlorine naturally.

Some cities add up to 4 parts per million of chlorine in tap water.

You can evaporate chlorine faster by heating your tap water before letting it sit in an open area. Ensure the container is wide to allow the water to aerate.

Place the water in a sunny spot for the sun’s UV rays to break down the chlorine, and use aquarium air stones to gas-out the chlorine in less time.

At What Temperature Does Chlorine Evaporate from Water

Chlorine will evaporate from water at room temperature but dissipates much quicker in heat. It could take up to 24 hours at room temperature to evaporate chlorine, but half that time if you boil tap water prior.

For this reason, I recommend you boil your tap water first, then let it sit for 2 to 3 hours (or so) before you add it to your fish tank. 2ppm of chlorine will take 5 days to evaporate, but if heated, the same amount will dissipate in less than 2 hours.

That’s all for this post!

Have fun keeping fish in a clean, chlorine-free tank.

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