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How Long Does It Take For Chlorine To Evaporate From Water? Full Answer Here

When planning to set up an aquarium, you’re most probably thinking of filling it with tap water. A certain level of chlorine is present in your tap water to disinfect it. But for your pet fish, this level can already be harmful or stressful. One way to remove this chlorine content is to let your water sit.

How long does water need to sit to dechlorinate? Generally speaking, water needs to sit for at least 24 hours to dechlorinate. It can take up to 5 to 6 days for the water to dechlorinate completely. Keep reading to learn more.

How Long Does Water Need To Sit To Dechlorinate?

Can You Dechlorinate Water By Letting It Sit?

Letting the water sit is one effective way of dechlorinating it. This method, although rather time-consuming, is one of the safest methods of dechlorinating water. This also makes dechlorinating a huge volume of water possible without any conscious effort to add heat.

How Long Does Water Need To Sit To Dechlorinate?

Water needs to sit under ultraviolet (UV) exposure for at least 24 hours for it to dechlorinate. Exposing the tap water to UV rays will allow the chlorine to escape the water naturally by evaporation. This is one of the easiest and most natural methods of dechlorinating tap water.

Doing this is recommended when dealing with larger volumes of water since this does not require as much work.

Increasing the temperature of the tap water also increases the rate at which it dechlorinated. An increase in the temperature of the water also means that the gas molecules trapped in the water would move faster and eventually escape into evaporation. Since chlorine is highly volatile, or in other words, easily evaporable such a process would work well for dechlorination.

How Long Does It Take Tap Water To Be Safe For Fish
How Long Does It Take Tap Water To Be Safe For Fish

How Long Does It Take Tap Water To Be Safe For Fish?

The time tap water takes to dechlorinate to be safe for fish would depend on the volume of water being used for the tank and the method being used to dechlorinate the water. But generally speaking, dechlorinating tap water to a level that is safe enough for fish should not take less than 24 hours.

To ensure all the chlorine content in the water has been removed through evaporation, you might have to wait for as long as 5 to 6 days.

Another way to dechlorinate tap water quickly is by adding chemicals and additives to it. But when doing this, you have to make sure that the chemicals you are adding are not harmful both to your fish and to the quality of the water to be used for these fish.

Other ways to speed up the dechlorination of tap water are listed below.

3 Ways to Dechlorinate Water

There are many other simple and effective ways to dechlorinate your water. Three of these methods include boiling the water, treating the water with Vitamin C, and exposing the water to U V light.

Boil the Water

Cooler water contains more gases. Boiling the water is a fast method that allows these trapped gases to escape and thus makes chlorine evaporate. Boiling decreases the holding capacity of dissolved or trapped gases within the water. Also, this is effective in removing chloramine, which is another disinfectant that is toxic to fish.

You can simply boil the water you want to place in your aquarium for 20 minutes over your stove to remove all the chlorine. After having boiled the water, just do not forget to let the water cool down before pouring it into the aquarium.

But chloramines are much less volatile, so boiling to remove these would have to take a longer time. The recommended time for this is about one hour. To increase the effectiveness of dechlorinating water by boiling, the surface area of the water must be greater.

This means that boiling water in a wide pot would be more effective than when this water is confined in a narrow column.

Treat the Water with Vitamin C

Adding Vitamin C to the water is the safest chemical method you can do in removing chlorine content. Vitamin C, which can either be ascorbic acid or sodium ascorbate can come in the form of powder or tablets. Adding Vitamin C neutralizes the chlorine and chloramine present in the water in a rather short amount of time. This neutralizing reaction happens rather quickly.

Adding large amounts of Vitamin C to the tap water can slightly lower its pH level but does not lower dissolved oxygen in the water as much as other sulfur-based chemical treatments do. Also, Vitamin C at levels high enough for dechlorination is not harmful to the fish.

Both forms of Vitamin C (ascorbic acid and sodium ascorbate) are stable and have a shelf life of at least one year when kept in a place that is cool and dry. These reasons make Vitamin C a popular chemical method of dechlorination compared to the other agents that are rather harmful to both the quality of water and the fish living in the water.

You can simply add a tablespoon of Vitamin C for every gallon of tap water you wish to pour into your aquarium. This means having to add 10 tablespoons of Vitamin C to 10 gallons of tap water in order to dechlorinate it.

To be more precise, every 100 gallons of water will need 1 gram of Vitamin C to neutralize 1 milligram per liter of chlorine. Simply put the neutralization process involves one part of chlorine and around 2.5 parts of Vitamin C.

When in powder form, adding this can be done by simply sprinkling it on the surface of the water and stirring it gently. It will precipitate chlorine to the surface since it is not soluble in water.

Expose the Water to Ultraviolet Light
Expose the Water to Ultraviolet Light

Expose the Water to Ultraviolet Light

Another effective but rather expensive method in dechlorinating water is installing a UV sterilizer for your tank. This piece of equipment would bombard your water with UV rays, which are effective in removing both chlorine and chloramine.

This UV method of dechlorination has become increasingly popular because of the minimal maintenance it requires and the absence of harmful chemicals in its use. Such kind of treatment reduces both chlorine and chloramine into byproducts that can be easily removed.

The level of UV light needed for dechlorination would depend on the volume of water to be used in the tank. How long the dechlorination process would take is also dependent on the strength of this UV light and the amount of organic matter present in the water.

But in normal cases, the UV light to be used for this method to be effective must have a wavelength of 254 nanometers (nm) and a radiant density of 600 milliliters (mL). You do not have to worry much about what these specifications mean as long as the UV sterilizer satisfies them.

Even if this method is relatively more expensive than the other mentioned methods, UV sterilizers are fortunately not too expensive. High-quality ones can be bought for as low as $150. A cheaper alternative version of this ultraviolet exposure method would be to leave the water outside under the sun for 24 hours.

In this method, the chlorine would naturally evaporate. This method is much easier, but just be careful as to where you would leave the water since other contaminants might unknowingly enter it.

Does Aerating Water Remove Chlorine?

Aside from the methods of dechlorinating water listed above, you can consider aeration as one of your choices. Aeration is another effective method that removes chlorine from water. Aeration is an important process that ensures constant water circulation.

This mechanical method not only ensures that oxygen is present within the water but also speeds up the escape of chlorine from the water through evaporation. A tank that is constantly aerated means that you will not have to worry much about high levels of chlorine in the water of your fish tank.

Aeration may involve two states of matter. It can either involve the liquid state, which manifests in the form of water, or the gaseous state, which manifests in the form of oxygen. Both of these states of matter have rather loose molecular structures and thus, although not immediately, are capable of mixing together.

When an air bubble is released underwater, water exerts pressure on all sides of the bubble and forces the bubble to take a spherical shape. This bubble would then rise to the surface and burst, releasing the air into the atmosphere. This bubble would float because air has less density than water.

Also, since chlorine readily evaporates, it can easily join this bubble as it goes out to the surface and gets released into the atmosphere. This is why aeration can speed up dechlorination.

There are different types of methods you can use to aerate the water.

Aeration Pump

A common and versatile device used to aerate water is the aeration pump. While oxygen can be absorbed by the surface of the water in the aquarium, it may not be enough for the fish. To address this, the pump mechanically forces air from the atmosphere into the water in the aquarium. Through this, oxygen is brought into the water for the fish to breathe.

Also, the oxygen forced into the tank will cause bubbles to rise from the water, and the chlorine dissolved with the water would escape along with these bubbles. Chlorine would easily do this since it is rather volatile.

With these bubbles constantly breaking the surface tension and causing water movement from the bottom to the top of the tank, the exchange of gases in the tank is sped up, and this includes the escape of chlorine into the atmosphere. The size of the pump would depend on the volume of water present in the tank.

Aerating Sprayer

The aerating sprayer is another effective way of aerating water in a fish tank in order to dechlorinate it. A spraying nozzle can be connected to a hose to spray water into the fish tank. Although chlorine itself is volatile and thus may easily evaporate by itself, aerating the water by an aerating sprayer can help speed up this process.

Overhead Filter

Another device that can be used for aeration is the overhead filter. The overhead filter normally uses a powerhead to work the filter. The mechanism of this powerhead pumps water from below the tank up to the overhead chamber.

After having flowed through the filter, the now filtered water would return to the tank. This process agitates the water surface by dropping a few inches, making the overhead filter probably better than the air pump when it comes to aerating the water.

Normally, the overhead filter can involve two outlets. The first outlet would go up the overhead chamber. As for the second outlet it goes straight out the nozzle and onto the surface horizontally. Greater agitation is caused by the latter outlet because it shoots water directly onto the surface and creates strong waves at the top.

This system involves the overhead chamber and the nozzle, two sources of aeration that make the process even more efficient. Just like the other mentioned devices used to aerate the water, dechlorination happens because of the agitation that occurs in the water.

This speeds up the process in which chlorine that is dissolved in the water escapes all the way to evaporation. However, generally speaking, aeration does not really work for chloramines, which are relatively less volatile. To address this, a dechlorination agent may accompany your aeration devices.

Dechlorinate Water in aquarium
Dechlorinate Water in aquarium


Dechlorination is an important process that ensures that the chlorine level of the tap water to be used in the aquarium would be safe enough for fish. The time that tap water takes to dechlorinate would ultimately depend on the volume of water and the method being used, although it can go for at least 24 hours or ensure total removal, as long as 4 to 5 days.

Feel free to drop your comments below if you have any relevant questions.

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