How To Dechlorinate Water For Fish: The Easiest Methods

Young man changing water in aquarium using siphon
Young man changing water in aquarium using siphon
Dr. Mollie Newton
Published by Dr. Mollie Newton PHD| Senior Editor
Last updated: July 7, 2024
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Although humans can drink tap water without any risk, it poses significant dangers to aquatic life. Tap water from urban sources often contains harmful chemicals such as chlorine and chloramine, which can be lethal to these creatures even in small quantities. Witnessing your fish become ill or die is a distressing experience for anyone.

Article Summary

  • Tap water contains chlorine and chloramine, which can be toxic to fish and aquatic life, and it should be dechlorinated before using it in an aquarium.
  • It’s important to know the difference between chlorine and chloramine and how to remove both from tap water.
  • There are various methods to dechlorinate water for fish, including using water conditioners, boiling, activated carbon, UV sterilization, reverse osmosis filtration, vitamin C treatment, and letting the water sit in sunlight.

Before you add tap water to your fish tank, you’ll need to dechlorinate it beforehand to ensure its safety. This might seem like a pretty complicated task, but don’t worry, it’s actually pretty easy!

Read on as I’ll be explaining how to dechlorinate water for fish in this article to keep your aquarium inhabitants happy and healthy.

Does My Tap Water Contain Chlorine?

Your water treatment center adds chlorine to all tap water to remove dangerous bacteria like E. Coli and other harmful contaminants, ensuring its safety for human consumption.

Despite being safe for humans to drink, it should never be used for aquatic life.

Man operating a faucet, pouring water into a pan
Tap water is great for us but not so much for fish

That’s why it’s crucial that you dechlorinate your tap water before you use it in any enclosure containing fish or aquatic creatures.

Not sure how to dechlorinate water to make it safe for your scaly friends? Don’t worry, I’ll be explaining the entire process later on!

How Do You Measure Chlorine?

You can measure chlorine in your tap water by using a chlorine test kit, which are available as test strips and liquids.

Alternatively, you can use a chlorine meter tester designed for pools and hot tubs.

However, this device can be a little pricey and may not be the most cost-effective method of measuring chlorine levels in your aquarium unless you own multiple fish tanks.


Testing your tap water can help you check how much chlorine it contains, allowing you to choose the right dechlorination method. Chlorine tests can also be used to check whether other sources of water, such as certain brands of bottled water, contain chlorine.

Why Is Chlorine Harmful to Ponds and Fish?

Chlorine is dangerous to ponds and fish as it reacts with living tissues and organic matter, which results in cell death (acute necrosis).

Fish gills are highly sensitive and are subjected directly to their underwater environment, so gill necrosis will often lead to suffocation and respiratory issues.

Depending on the amount of chlorine a fish is exposed to, chlorine poisoning can cause death within a few hours or even a few minutes.

If your fish is subjected to too much chlorine, they may display symptoms such as pale coloration, excessive mucus on the body, erratic swimming, hyperemia (redness) on the body, rapid breathing, and hovering near the water surface.

Can Fish Survive in Chlorinated Tap Water?

Fish cannot survive in chlorinated water and will either die slowly or straight away depending on the level of chlorine present in the water.

Chlorine kills off living cells and will severely burn a fish’s gills and skin.

Alcohol being poured into a fish tank with fish in it
You might as well be pouring alcohol into your fish tank!

Even if a fish is only subjected to chlorinated water for a short period of time, it can cause irreversible damage to a fish’s body and organs.

You should always ensure your tank water is free of chlorine and chloramine using either a water conditioner or a natural method such as activated carbon (more on this later!).

How to Dechlorinate Water For Fish Tank

The most common way on how to dechlorinate water for fish is by using an aquarium water conditioner, which neutralizes chlorine, chloramine, and heavy metals from tap water to make it safe for fish to inhabit. You can find this product at your local fish store or online.

Water conditioners usually contain sodium thiosulfate, which converts chlorine and chloramine into harmless by-products.

There are other ways to dechlorinate tap water, including boiling water, using activated carbon, adding ascorbic acid, and using a reverse osmosis system.

Natural Dechlorinating Methods (No Chemicals)

If you’d rather avoid adding a chemical water treatment to your tap water, there are plenty of natural dechlorination methods, which I’ll be covering below.

Let It Sit

One of the simplest ways to dechlorinate tap water naturally is to let water sit in direct sunlight for at least 24 hours in an open container, ideally with an air pump or an air stone – the air bubbles will help quicken the process. The volatile chlorine will begin to evaporate from water when it is exposed to air.

It’s important to note that this method only removes chlorine, not chloramine, which is another disinfectant added to tap water.  If chlorine is the only chemical in your water, you can stop here.

Unfortunately, unlike chlorine, chloramine is highly soluble in water and can only be eliminated with an aquarium water conditioner or other natural methods, which I’ll be going over below!

Using UV Light

Some sources claim that Ultraviolet light can remove both chlorine and chloramine from your tap water by using a UV sterilizer.

This device helps eradicate harmful bacteria, free-swimming algae, viruses, and other nuisance microorganisms from your fish tank water.

A closeup of an aquarium cleaning device
A close-up of a UV sterilizer(1)

However, there is insufficient information on UV light source effectiveness in an aquarium setup (most of the research on this topic centers around food and Pharmaceutical industries), especially as you’d need an extremely high dosage of UV light.

… you’d need an extremely high dosage of UV light.

The majority of commercial aquarium UV sterilizers will not output enough UV light to eliminate either chlorine or chloramine.

Personally, I wouldn’t risk using an UV sterilizer for the sole reason of dechlorinating water – there are much more effective and safer ways to do so.

That being said, UV sterilizers are fantastic for improving the water quality of your tank water, particularly in ponds and marine tanks, as well as aquariums that house sensitive species of fish or inhabitants that require pristine water conditions.

Boiling Water

Another straightforward (albeit slightly time-consuming) natural way to remove all the chlorine and chloramine tap water is to boil it.

The heat and aeration formed by boiling the water will get rid of both chlorine and chloramine.

All you need to do is boil tap water for at least 20 minutes continuously in a pan on a stove top. Let water cool completely before adding it to your aquarium.

Depending on how large your tank is, the boiling method could take a while, but it’s an easy and cheap way to neutralize chloramine and chlorine!

Carbon Filter / Activated Carbon

You can treat your fish tank with activated carbon to remove chlorine alongside chloramine from your fish tank, in addition to odors, tannins, chemicals, and inorganic and organic compounds from your water.

Close up of carbon filter granules
Granules like these are used in carbon filters(2)

You can purchase simple carbon filter cartridges for your tank, which you can then place inside your aquarium filter.

Bear in mind that carbon needs to be replaced once a month. Carbon gradually becomes saturated, which means it will no longer be able to eradicate decontaminants and chemicals from your water.

Reverse Osmosis Filter

Reverse osmosis (RO) is a water purification process used by some aquarists to remove all chemicals and impurities from their tap water, including chlorine, chloramine, and dissolved salts.

RO water contains no water hardness and has neutral pH levels.

Reverse osmosis systems can be installed underneath your kitchen sink or wherever water supply enters your house. However, they are expensive, so this option isn’t the most affordable way of dechlorinating water. Additionally, RO water needs to be remineralized before it is safe for fish.

Vitamin C Treatment

Using a vitamin C treatment is commonly used to naturally dechlorinate tap water.

The method is a simple and cheap solution – just add vitamin C in the form of ascorbic acid to your tap water to start removing chloramine and chlorine.

You can buy vitamin C in this form in most health stores or online.


You’ll need to add one teaspoon of ascorbic acid for every gallon of water. So, for example, if your tank has a volume of 20 gallons of water, you’ll need 20 teaspoons.

Bottled Water

There are a few different types of bottled drinking water: spring water, filtered water, and purified water.

Spring water is the safest choice, but you’ll need to check its pH, hardness, and the type of minerals, heavy metals, and nutrients it contains beforehand.

Using Chemicals to Dechlorinate

A chemical water treatment is the quickest and easiest way to eliminate chloramine, chlorine and other chemicals from tap water to ensure its safety for fish.

As mentioned earlier, chemical water or tap water conditioners usually contain a blend of chemicals, but primarily sodium thiosulfate to convert chlorine and chloramine into less harmful forms.


There are many chemical solutions on the market, but I personally use Seachem Prime in all my fish tanks. It eradicates heavy metals, chloramine, and chlorine from the water, and detoxifies ammonia, nitrite, and nitrate. It’s also non-acidic and won’t affect pH levels in your aquarium water!

The important thing to know is when to add the dechlorinator. The video below goes over when to do so when changing out your tank’s water.

How to properly and easily use dechlorinators

Use Caution When Using Chemicals

While water conditioners are one of the most effective ways to remove chlorine and chloramine from your tap water, it’s important not to overdose on the treatment, otherwise it could be harmful to your fish and also damage beneficial bacteria in your fish tank’s biological filter system.

Make sure you know how much water your fish tank can hold so you can dose the product correctly.

The dosage depends on the concentration of the product. For instance, Seachem Prime is a concentrated water conditioner, so you only need to use a small amount (5ml per 50 gallons) to treat your water.

Most aquarium conditioners will list the recommended dosage per gallon of water on the back of the label. Be sure to read over the label to ensure you’re using the right dose.

Chlorine vs Chloramine: Know the Difference

While they sound pretty similar with both added to tap water to disinfectant, there are actually a couple of differences between the two.

Chlorine is a gas that is dissolved in drinking water and pool water to eradicate bacteria and other microorganisms.

Chlorine naturally evaporates from water when it is exposed to air for at least 24 hours. After this time, it’s going to be chlorine-free water.

Chloramine, on the other hand, is a variant of chlorine that contains ammonia. It does not evaporate from water and can only be removed using water conditioners, activated carbon, reverse osmosis filtration, and vitamin C.


What If I Need to Dechlorinate in a Hurry?

If you need to remove chloramine and chlorine in the water quickly, a chemical dechlorinator is your best option. Water conditioners will eliminate these two substances as soon as they are added to tap water.

Three bottles of water conditioners
These are the types of conditioners you need for your tank(3)

Should I Buy Dechlorinated Water?

Most fish stores sell special “betta water” or bottled water designed especially for fish, but these products are usually just a marketing ploy and are nothing more than dechlorinated water.

While you can use these types of water, it’s much cheaper to dechlorinate tap water yourself using a water conditioner or one of the natural methods mentioned earlier in the article.

Should I Buy Dechlorinated Water?

Most pet stores sell special “betta water” or bottled water designed especially for fish, but these products are usually just a marketing ploy and are nothing more than dechlorinated water.

While you can use these types of water, it’s much cheaper to dechlorinate tap water yourself using a water conditioner or one of the natural methods mentioned earlier in the article.

Is There a “Best Water” to Use for Aquariums?

There isn’t really a “best water” for aquariums – water ideal for fish is that which is free from chlorine, chloramine, toxic ammonia, nitrite, and excess levels of nitrates.

However, all species of fish have preferred water parameters, which includes differing levels of temperature, pH, and water hardness.


Ensuring your fish tank is properly cycled before you add any livestock and regular water changes will help you maintain zero levels of ammonia and nitrite, as well as keep nitrate levels in check.

Final Thoughts

I hope this article helped you understand the importance of using dechlorinated water for fish, as well as some of the best methods for removing chlorine and chloramine from your tap water.

Various Options

You can treat tap water with a water conditioner, activated carbon filter, vitamin C, reverse osmosis system, or let it boil for 20 minutes.

Remember, simply leaving an uncovered water for 24 hours will only cause the chlorine to evaporate, not chloramine – you’ll need to use one of the aforementioned methods to remove this substance.

Water Maintenance

Make sure your tank is fully cycled before you add any fish, and keep on top of weekly water changes to prevent a cloudy fish tank and the buildup of ammonia, nitrite, and nitrate.

What’s your preferred dechlorination process? Let me know on our social media platforms and be sure to share this post with friends, family, or other fellow aquarists!

Check out my other guides on all things fish to make sure you provide the best care for all your scaly friends.

(1) Aquarium UV sterilizer on a white background by Marco Verch Professional Photographer – licensed under CC BY 2.0
(2) Small activated carbon granules, used in purification for water filters by Marco Verch Professional Photographer – licensed under CC BY 2.0
(3) I swear! It’s for my fish!! by Moto “Club4AG” Miwa – licensed under CC BY 2.0

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