When setting up or maintaining an aquarium, few tasks are as important—and often overlooked—as properly treating tap water before it enters your tank. While tap water is perfectly safe for human consumption, it can contain substances that are harmful, and sometimes deadly, to fish, shrimp, plants, and the beneficial bacteria that keep your aquarium healthy.
This is why water dechlorination is an essential part of aquarium maintenance. Whether you’re performing a weekly water change, topping up evaporated water, or setting up a brand-new tank, removing chlorine and other harmful chemicals should always be a priority.
In this guide, we’ll explain what chlorine and chloramine are, why they are added to tap water, and how proper dechlorination can protect your aquarium and its inhabitants.
What Is Chlorine?
Chlorine is a chemical disinfectant added to public water supplies to kill harmful bacteria, viruses, and other microorganisms.
Water companies use chlorine to ensure tap water remains safe as it travels through miles of pipes before reaching your home.
While this is excellent for human health, chlorine can be extremely dangerous to aquarium life.
Even relatively low levels of chlorine can damage fish gills, stress aquatic animals, and harm the beneficial bacteria that power your aquarium’s biological filtration.
What Is Chloramine?
Many water companies now use chloramine instead of chlorine, or alongside it.
Chloramine is a combination of:
- Chlorine
- Ammonia
It is more stable than chlorine and remains active in the water supply for longer periods.
While this makes it effective for water treatment, it also makes it more difficult to remove.
Unlike chlorine, which can dissipate naturally over time, chloramine requires specific treatment to neutralise it safely.
Why Is Chlorine Harmful to Fish?
Fish absorb oxygen directly through their gills.
Unfortunately, chlorine attacks delicate gill tissues and can cause:
- Respiratory stress
- Gill damage
- Reduced oxygen uptake
- Increased susceptibility to disease
- Death in severe cases
Because fish are constantly exposed to the surrounding water, even small amounts of chlorine can quickly become problematic.
Chlorine Can Harm Beneficial Bacteria
Many aquarium hobbyists focus on protecting their fish, but chlorine also poses a serious threat to the beneficial bacteria living in your filter.
These bacteria are responsible for:
- Breaking down ammonia
- Processing nitrite
- Supporting the nitrogen cycle
Without these bacterial colonies, toxic waste products can quickly build up.
Adding untreated tap water during a water change can damage or destroy beneficial bacteria, potentially leading to ammonia spikes and water quality issues.
The Risks of Chloramine
Chloramine presents additional challenges because it contains ammonia.
When chloramine is broken down, it releases:
- Chlorine
- Ammonia
Quality water conditioners neutralise both components, making them safe for aquarium use.
Failure to properly treat chloramine can result in:
- Fish stress
- Water quality problems
- Biological filtration disruption
This is why understanding your local water supply is important.
Can You Leave Water Standing to Remove Chlorine?
Many older aquarium guides recommend leaving tap water in a bucket for 24–48 hours to allow chlorine to evaporate naturally.
While this can help remove some forms of chlorine, it has significant limitations:
- It does not reliably remove chloramine
- It can be inconsistent
- It requires planning ahead
- It offers no protection against heavy metals
Modern water conditioners provide a much safer and more reliable solution.
What Is a Water Conditioner?
A water conditioner, sometimes called a dechlorinator, is a treatment that neutralises harmful chemicals in tap water.
Most modern water conditioners:
- Remove chlorine
- Neutralise chloramine
- Detoxify heavy metals
- Protect fish during water changes
They work almost instantly and make tap water safe for aquarium use.
Why Every Aquarium Needs a Water Conditioner
Regardless of whether you keep:
- Tropical fish
- Goldfish
- Shrimp
- Planted aquariums
- Marine fish
- Reef aquariums
a water conditioner should be considered essential equipment.
It’s one of the simplest and most affordable ways to protect your aquarium from preventable problems.
Additional Benefits of Water Conditioners
Many premium water conditioners do more than simply remove chlorine.
Additional benefits may include:
Stress Reduction
Some conditioners contain ingredients that help support fish slime coats and reduce stress.
Heavy Metal Detoxification
Tap water can occasionally contain trace amounts of metals such as:
- Copper
- Zinc
- Lead
Quality conditioners help neutralise these substances.
Ammonia Detoxification
Certain products temporarily detoxify ammonia, providing an additional layer of protection during maintenance.
Popular Water Conditioners
Several excellent dechlorination products are available to aquarium hobbyists.
Popular options include:
Seachem Prime
Widely regarded as one of the most effective water conditioners available.
Benefits include:
- Removes chlorine and chloramine
- Detoxifies ammonia
- Concentrated formula
- Suitable for freshwater and marine aquariums
Fritz Complete
A highly effective conditioner that treats:
- Chlorine
- Chloramine
- Heavy metals
Suitable for both freshwater and marine systems.
API Tap Water Conditioner
An easy-to-use option popular with beginner aquarists.
NT Labs Tap Safe
A trusted water treatment solution widely used in the UK aquarium hobby.
Dechlorination During Water Changes
Water changes are one of the most important aspects of aquarium maintenance.
Every time you add fresh tap water, you should:
- Measure the required amount of conditioner.
- Add it according to the manufacturer’s instructions.
- Ensure the water is fully treated before exposing fish to untreated water.
Many aquarists add conditioner directly to the aquarium during water changes, while others treat replacement water separately.
Both methods can be effective when performed correctly.
What About Marine Aquariums?
Marine aquariums are equally vulnerable to chlorine and chloramine.
In fact, many reef keepers go a step further by using:
- Reverse Osmosis (RO) water
- Deionised (DI) water
These systems remove virtually all impurities before salt is mixed.
Even when using RO water, dechlorination remains important if untreated tap water is used at any stage.
Do Planted Aquariums Need Dechlorination?
Absolutely.
Although plants may tolerate low levels of chlorine better than fish, untreated water can still:
- Damage plant tissues
- Harm beneficial bacteria
- Stress aquatic livestock
Every planted aquarium should receive proper water treatment.
What Happens If You Forget?
Accidentally adding untreated tap water doesn’t always result in immediate disaster, but it can cause:
- Fish stress
- Bacterial colony damage
- Increased disease risk
- Water quality instability
If you realise you’ve forgotten to add conditioner, treat the water as quickly as possible and monitor livestock closely.
Is Bottled Water Better?
Many people assume bottled water is safer than tap water.
However, bottled water often contains:
- Unknown mineral content
- Variable pH levels
- Inconsistent hardness
For most aquariums, properly treated tap water is a better and more economical option.
Understanding Your Local Water Supply
Water quality varies depending on location.
It’s worth checking with your local water provider to determine:
- Whether chlorine or chloramine is used
- Water hardness levels
- Other relevant water chemistry information
This knowledge can help you make informed decisions about aquarium maintenance.
Common Dechlorination Myths
“My Fish Look Fine Without It”
Damage from chlorine can occur even when symptoms are not immediately obvious.
“I Only Added a Small Amount of Water”
Even small top-ups can introduce chlorine into the aquarium.
“My Filter Removes Chlorine”
Standard aquarium filters are not designed to remove chlorine effectively.
“Natural Evaporation Removes Chloramine”
It generally does not.
Using a dedicated water conditioner remains the safest approach.
The Cost of Not Dechlorinating
Water conditioner is one of the least expensive aquarium products you will buy.
Compared to the cost of:
- Fish
- Corals
- Plants
- Aquarium equipment
it’s a tiny investment that provides enormous protection.
Skipping dechlorination simply isn’t worth the risk.
Final Thoughts
Water dechlorination is one of the most important yet straightforward aspects of aquarium care. Chlorine and chloramine are added to tap water to make it safe for humans, but they can be highly damaging to fish, plants, invertebrates, and beneficial bacteria. By using a quality water conditioner every time you add tap water to your aquarium, you can protect your livestock, maintain biological filtration, and ensure a healthier aquatic environment.
Whether you’re caring for a freshwater community tank, a planted aquascape, a marine fish-only system, or a thriving reef aquarium, proper dechlorination should always be part of your maintenance routine.
At Charterhouse Aquatics, we stock a wide range of trusted water conditioners and aquarium treatments to help keep your fish healthy and your water quality at its best.
