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You’ve set up your aquarium and chosen your accessories, but you still have an important task to take care of—preparing the water for its future inhabitants. Your aquatic life’s water is like the air you breathe, and it needs to be free of contaminants so your fish and other creatures can thrive. That’s why learning how to dechlorinate tap water is essential for those who want to take care of fish.   

Is chlorine bad for fish? 

Yes, chlorine is bad for fish. It can cause chemical burns around their gills and even be absorbed into the bloodstream, where it can lead to further damage throughout your fish’s bodies.  

Their bodies won’t function properly if they can’t swim in dechlorinated water, and the stress that can cause may worsen health conditions. If your fish are darting around the aquarium at random or seem to be gasping for air, that could be a sign that you have chlorine or chloramine in your aquarium water.  

Does my tap water contain chlorine? 

Nearly all municipal water contains either chlorine or chloramine, so it’s generally safe to assume that you’ll need to dechlorinate your tap water before adding it to your aquarium (unless you have a well).

Chloramine—which is chemically bonded chlorine and ammonia—is often added to municipal water instead of chlorine because it breaks down more slowly and provides protection for a longer period.  

It’s also more difficult to remove from water than chlorine because the chlorine and ammonia must be separated from each other before they can be removed from the water.  

Do I need to dechlorinate saltwater? 

Yes, because most saltwater aquarists create their own water by adding reef salt to a freshwater base. Many saltwater aquarium keepers use reverse osmosis to remove chlorine as it provides the most neutral water free of other toxins and dissolved solids.  

How to dechlorinate water 

No matter which chemical composition your locality adds to your water, you’ll need to remove it before filling your aquarium. Here are the some of the most popular ways to provide your aquatic life with healthy water. 

Water conditioners

Water conditioners break the bond between chlorine and ammonia—if chloramine is present—then remove both toxins from the water. This dechlorination method is usually the easiest.  

Water conditioners are available at most Petco pet care centers. Look for conditioners that work on chlorine, chloramine and ammonia, especially if you’re unsure what your water contains. Read the instructions carefully, as adding too much of some types of water conditioners to your aquarium can disrupt the ecosystem as much as the chlorine itself.  

Bottled water

Want to avoid treating your water altogether? Many pet and fish stores sell UV-purified bottled water that is already free of chlorine, chloramine and ammonia, and it often contains electrolytes. These bottles range from .25 to 2.5 gallons or more. While purified water is a popular option for dechlorinating water for betta fish, it’s usually cost-prohibitive for those with larger aquariums. 

Note: Bottled drinking water intended for human consumption is not always appropriate for fish. Distilled water can be particularly bad since the distilling process removes all minerals, including those that are good for your aquatic life and help stabilize the aquarium pH. Spring water may not be ideal, as its pH values can vary based on where it comes from. Always test the parameters first if you’re considering utilizing spring water. Many bottled drinking waters also contain small levels of chlorine.

Reverse osmosis

Reverse osmosis is one method employed by many professional fish keepers. This process forces water through a carbon filter and special membrane to separate contaminates from the water, including chlorine, chloramine and ammonia. It’s commonly used for drinking water, and reverse osmosis systems can even be installed directly underneath your sink so that dechlorinated water is readily available. 

Activated carbon filters

Activated carbon filters are commonly used to filter drinking water in the home. If you’ve ever used a Britta water pitcher, then you already have some experience with them. Activated carbon filters absorb many dangerous and undesirable chemicals in water, including chlorine and chloramine.

UV light

UV light can remove both chlorine and chloramine through photodegradation, the process by which light breaks down cells through oxidation and hydrolysis. At certain wavelengths, UV light can reduce both chlorine and chloramine into byproducts that can then be easily removed.  

Leave the water outside for 24 hours to allow the chlorine to evaporate, then place it underneath a UV light for another 24 hours to eliminate chloramine and any remaining chlorine. Most commercially available UV lights are only effective on small amounts of water—no more than 10 gallons at a time.  

Vitamin C

Two forms of vitamin C——ascorbic acid and sodium ascorbate—neutralize both chlorine and chloramine. Sodium ascorbate is a good choice for aquariums because it is neutral and shouldn’t affect the pH. Ascorbic acid, on the other hand, can lower pH.  

Be careful not to add too much, as it is technically a chemical and can harm your fish in large doses. The amount you’ll need depends on the amount of chlorine, the amount of water and the type of vitamin C you choose, so consult someone knowledgeable on the subject before moving forward with this method.  

Aeration or boiling

These last two methods for removing harmful chemicals from water are more effective on chlorine than chloramine, and aeration doesn’t eliminate chloramine at all. You’ll want as much surface area as possible when aerating water, so using a long, shallow container is recommended. You’ll need to leave the water out for 12 to 24 hours, depending on the amount of water you’re aerating and its chlorine content. When in doubt, consult an aquatic specialist.  

Boiling is effective against chloramine, but it will take as long as an hour or more. If you know that your water contains chlorine—not chloramine—you can boil about 10 gallons of water for 10 minutes to neutralize it. For larger amounts, extend the time to 15 to 20 minutes. Once your chlorine-free water has cooled down, test the temperature before adding it to your aquarium.  

Reviewed by Don Spaeth

Don is Petco’s National Aquatic Care, Education and Programs Manager. He is an avid aquarist who has worked with and cared for freshwater and marine aquatic life for over 40 years. Throughout his 27+ years with Petco, Don has actively been involved with our aquatic vendor partners and worked to promote aquatic education both in store and company-wide. 

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