Chlorine and Chloramine: Protecting Koi During Water Changes

Why drinking water contains chlorine or chloramine

Public water supplies are disinfected to control bacteria and other disease-causing microorganisms. Free chlorine is a highly effective disinfectant, but it can decay as water moves through pipes. Chloramine is produced when ammonia is combined with chlorine and is used by some utilities because it provides a more persistent disinfectant residual in distribution systems.

U.S. EPA guidance describes chloramines as longer-lasting secondary disinfectants. Malaysian drinking-water standards likewise include parameters for free residual chlorine, combined residual chlorine and monochloramine, showing that disinfectant residuals are an expected part of treated drinking-water management.

For human drinking-water safety, that residual is useful. For fish, it must be removed before exposure.

Chlorine and chloramine are not the same thing

How toxic is chlorine to fish?

The Merck Veterinary Manual states that chlorine and chloramine are highly toxic to aquatic organisms. It reports adverse effects from chlorine at concentrations as low as approximately 0.02 mg/L and mortality at about 0.04 mg/L under some circumstances.

These values are far below the disinfectant residuals that can be acceptable in drinking water for people. Merck's routine freshwater reference is therefore not a small allowable concentration - it is 0 mg/L for both total chlorine and free chlorine whenever live aquatic animals are present.

For koi, the correct target is not 'low chlorine'. The target is no detectable chlorine or chloramine.

Why koi gills are so vulnerable

The gill is designed to expose an enormous thin surface area to water so oxygen and ions can move efficiently between the environment and the bloodstream. That same design makes the gill highly vulnerable to oxidising chemicals.

Chlorine-based disinfectants can irritate and damage gill epithelium, disrupt normal gas exchange and injure mucus-producing surfaces. Severe exposure can therefore look like an oxygen emergency even when the water itself contains adequate dissolved oxygen.

Chlorine can also injure the skin, fins and corneal surfaces. Chronic low-level exposure may be less dramatic than a catastrophic overdose but can still produce persistent irritation and unexplained losses.

What chlorine or chloramine exposure can look like

Merck describes both acute and chronic presentations. Possible signs include:

• sudden respiratory distress or surface piping;
• rapid opercular movement and crowding near aeration or water returns;
• lethargy, irritation or abnormal swimming;
• excess mucus on the skin and gills;
• gill inflammation or necrosis in more severe exposure;
• ragged fins;
• cloudy corneas or irritated eyes;
• sudden deaths after a refill or water change;
• chronic unexplained mortality when small residuals repeatedly enter the system.

None of these signs is specific to chlorine. Nitrite poisoning, low dissolved oxygen, high carbon dioxide, parasites and other gill diseases can produce similar behaviour. The timing of the problem - especially immediately after adding municipal water - is an important clue.

Why a small untreated water change can still be dangerous

Keepers sometimes assume that a small percentage of untreated tap water will be diluted enough to become harmless. The arithmetic shows why this is a poor safety strategy.

Imagine source water containing 0.5 mg/L total chlorine. A 10% water change would have a theoretical immediate mixed concentration of 0.05 mg/L before accounting for reaction with organic material or any neutralisation. That simple dilution value is already in the same order of magnitude as concentrations associated with serious fish toxicity in veterinary references.

Real ponds have chlorine demand, so the residual does not remain exactly at the theoretical mixed concentration. But koi gills can be exposed during the reaction process. Dilution is therefore not a substitute for dechlorination.

The safest assumption is that every municipal-water change - large or small - requires proper treatment.

Free chlorine, combined chlorine and total chlorine

Understanding test terminology prevents one of the most common mistakes in chloraminated water.

Merck specifically warns that water treated mainly with chloramine can test negative for free chlorine while still showing a significant total-chlorine residual. Testing only free chlorine can therefore create a false sense of safety.

When municipal water is involved, test both free and total chlorine - not free chlorine alone.

How to recognise chloramine from test results

A simplified interpretation is useful:

Test-strip resolution can be poor at the very low concentrations important to fish. For serious koi systems, especially dealer facilities, use a method capable of detecting low residuals and verify the treated-water output rather than relying only on the dechlorinator dose calculation.

Why test immediately

Chlorine is reactive and can disappear after it has already caused damage. Merck notes that the chemical can be transient and that a negative test after an event does not completely exclude previous contamination.

If koi begin gasping immediately after a water change, collect and test samples as soon as possible from the source water and the pond. Waiting several hours can make the event harder to confirm.

Chloramine creates a second problem: ammonia

Chloramine is produced from chlorine and ammonia. When the chlorine component is neutralised, ammonia can be released into the water.

Merck specifically warns that repeated water changes using chloraminated water can cause ammonia accumulation if the keeper removes the chlorine portion but does not account for the ammonia that remains.

A mature biological filter may rapidly oxidise a modest ammonia input. A new, damaged, undersized or oxygen-limited biofilter may not. This is especially important in quarantine tanks, recently cleaned filters and dealer systems receiving large water changes.

Removing chloramine is not always the end of the water-change chemistry. You must also consider the ammonia that dechlorination can leave behind.

Why sodium thiosulfate works differently with chlorine and chloramine

Sodium thiosulfate is a well-established reducing agent used to neutralise chlorine. When it is used on chloramine, it can neutralise the chlorine component but the nitrogen portion remains as ammonia.

This is not a reason to avoid sodium thiosulfate. It is a reason to understand what happens next. In a mature system, the biofilter may handle the ammonia. In a new or vulnerable system, a product specifically labelled for chloramine and ammonia management may be more appropriate.

Always follow the exact product label. Dechlorinators differ in concentration and chemistry, so there is no universal number of millilitres that is safe to apply to every product or pond.

Do not guess dechlorinator dosage

Dechlorinator failure is often a calculation problem rather than a product problem. Common causes include underestimating the volume of new water, using the wrong product concentration, confusing gallons with litres, dosing for free chlorine when chloramine is present, or assuming the municipal residual is always the same.

Merck notes that water utilities may vary disinfectant treatment and that temporary higher residuals can occur during maintenance. A dose that worked last month should not be treated as proof that it will always be sufficient.

The safest commercial workflow is to know the actual incoming flow or refill volume, dose according to the product label and measured disinfectant, and verify zero residual at the point where treated water enters the fish system.

Should dechlorinator be added before, during or after the refill?

For stored-water preparation, the simplest approach is to treat the replacement water before it reaches koi, mix thoroughly and verify zero residual.

For direct-to-pond refilling, the dechlorinator must be present soon enough to neutralise disinfectant as the new water enters. Product instructions differ, so follow the label rather than using a generic sequence copied from another keeper.

Adding dechlorinator only after the pond has been filled exposes the gills during the entire refill and defeats the purpose of prevention.

Pre-treatment tanks: the most controllable approach

For dealers, high-value collections and large regular water changes, a dedicated treated-water reservoir can provide an extra layer of safety. New water can be conditioned, aerated, temperature checked and tested before it ever reaches koi.

A pre-treatment tank also makes it easier to confirm whether chlorine or chloramine is present, monitor any ammonia released from chloramine, and detect unexpected source-water changes.

The reservoir itself must be kept clean and should not become a stagnant organic-water store. Dechlorination does not guarantee that stored water remains biologically safe indefinitely.

Can you simply let tap water stand?

Standing or aerating water may reduce free chlorine over time because chlorine is reactive and can dissipate. The rate, however, depends on temperature, light, aeration, container geometry and chlorine demand.

Chloramine is specifically used because it is more persistent. U.S. EPA guidance states that allowing chloraminated drinking water to sit at room temperature should not be relied upon to remove monochloramine.

Therefore, 'leave it overnight' is not a reliable general dechlorination method unless you have tested the actual water and verified zero total chlorine before use.

Does boiling remove chloramine?

U.S. EPA guidance states that boiling should not be relied upon to remove monochloramine from drinking water. In koi management, boiling is obviously impractical for pond volumes anyway.

The broader lesson is that chloramine requires deliberate treatment rather than assumptions based on methods that work better for free chlorine.

Activated carbon can remove chlorine and chloramine - but design matters

Merck notes that activated carbon filters can be installed on incoming municipal water to remove chlorine and/or chloramine. This can be an excellent solution for continuous trickle-in systems, dealer facilities and large ponds.

However, carbon performance is not infinite. Removal depends on carbon type, bed depth, flow rate, contact time, water chemistry and how exhausted the media has become. Chloramine is generally more difficult to remove than free chlorine.

A carbon system should therefore be sized for the actual flow and disinfectant load, maintained on a planned schedule and verified with free- and total-chlorine testing at the outlet.

A carbon filter is not proof of safe water. Zero residual at the outlet is proof of safe water.

Why flow rate matters in carbon treatment

Water needs sufficient contact with the carbon surface for the chemical reactions and adsorption processes to occur. Running water through an undersized carbon cartridge too quickly can allow breakthrough even when the cartridge looks physically clean.

For a high-flow automatic top-up or dealer system, design should be based on verified treated-water flow rather than the nominal pipe size alone. Once the system is commissioned, test at the real operating flow.

Reverse osmosis is not a chloramine-removal strategy by itself

U.S. EPA guidance notes that reverse osmosis membranes should not be relied upon to remove monochloramine. Many practical RO systems use carbon pre-treatment specifically because disinfectants must be removed before the membrane and before the water reaches sensitive aquatic systems.

If RO water is used for koi, confirm both the pre-treatment of chlorine/chloramine and the later remineralisation of GH and KH. Producing very low-TDS water does not solve disinfectant safety automatically.

Can ultraviolet light remove chlorine or chloramine?

A UV steriliser is designed primarily to inactivate microorganisms passing through its chamber. It should not be treated as a general replacement for chemical dechlorination or properly designed carbon treatment.

If the water source contains disinfectant residual, use a method specifically validated for removing that residual and then verify with testing.

What about shower filters and aeration?

A Bakki Shower or heavy aeration can help drive off some free chlorine because they increase air-water contact, but chloramine is much more persistent. Neither should be your primary safety barrier when municipal water is entering a stocked pond.

It is much safer to neutralise or filter the disinfectant before the new water contacts the koi and biofilter.

Chlorine and the biological filter

Chlorine and chloramine do not distinguish between unwanted microbes and the nitrifying microorganisms you rely on inside the biofilter. Merck notes that municipal disinfectants are toxic to the necessary bacteria in aquatic systems as well as to the fish.

A significant untreated-water event can therefore create two phases of trouble. The first is direct gill and skin injury. The second can appear later if the biofilter has been damaged and ammonia or nitrite begins to accumulate.

After any known or suspected chlorine/chloramine exposure, continue testing TAN and nitrite for several days even if the fish appear to recover.

Why new ponds and quarantine tanks are at greater risk

A mature biofilter may be able to handle some ammonia released during chloramine dechlorination. A new pond, quarantine tank or recently disinfected filter has less biological capacity.

That makes source-water management especially important during cycling. Repeated large chloraminated-water changes can keep adding ammonia while the biofilter is still immature.

Test TAN after major water changes in vulnerable systems rather than assuming that a conditioner has made every consequence disappear.

The hidden risk of automatic top-up systems

Automatic top-up is convenient, but an uncontrolled top-up connected directly to municipal water can become a continuous poisoning system if a valve sticks open or a dechlorination stage fails.

High-value koi systems should use safeguards such as metered flow, timers or maximum-run controls, leak alarms, verified carbon capacity or dosing equipment, and regular residual-chlorine testing.

Design the system for failure. Ask what happens if the float valve sticks for six hours, the carbon becomes exhausted or the dosing pump loses power.

Continuous trickle-in water: safe only when treatment is equally continuous

A continuous low-rate water exchange can be excellent for koi water quality, but the incoming stream must be continuously safe. The small flow rate does not make chlorine harmless.

For trickle systems, activated carbon or a precisely controlled dechlorination system is often more practical than manually dosing the pond every day. Whichever method is chosen, verify the outlet under real operating conditions.

Source-water chemistry can change without warning

Municipal water is not chemically identical every day. Utilities may alter treatment in response to source-water conditions, network maintenance or microbial control needs. Merck specifically notes that temporary boluses of chlorine or chloramine can occur in some distribution systems during maintenance protocols.

For this reason, the source-water test you performed months ago is useful background information but not permanent proof. Dealer systems and large automated ponds should test periodically and after any utility notice or unexplained change in water smell, taste or residual readings.

Malaysia-specific considerations

Malaysia's published drinking-water standards include free residual chlorine, combined residual chlorine and monochloramine parameters. That is a reminder that treated tap water may contain a disinfectant residual that is entirely appropriate for human drinking-water safety but unacceptable for koi.

The actual disinfectant and residual can vary by treatment plant, distribution zone and operating conditions. Do not assume that all Malaysian tap water is free chlorine, and do not assume that all of it is chloraminated. Test the water that actually enters your pond.

For Malaysian dealers who perform frequent high-volume water changes, the safest long-term approach is a verified treatment system - for example a correctly designed carbon stage, chemical dechlorination, or both - combined with routine free- and total-chlorine testing.

Warm tropical water also means high koi metabolism and high biofilter demand. If chloramine treatment releases ammonia into a heavily stocked system, the filter may already be operating close to peak capacity. This makes TAN monitoring after large water changes particularly useful.

A safe water-change workflow

1. Know your source. Determine whether municipal water contains free chlorine, chloramine or both by testing free and total chlorine.

2. Calculate the real replacement-water volume. Do not guess from pond dimensions if the system has sumps, filters and pipework.

3. Choose a treatment method designed for the actual disinfectant. If chloramine is possible, use a method that specifically addresses chloramine.

4. Treat before or as the new water enters so koi are never exposed to an untreated stream.

5. Mix thoroughly and verify zero detectable free and total chlorine at the point of fish exposure.

6. If chloramine is present, consider the ammonia released by dechlorination and check TAN in new, damaged or heavily loaded systems.

7. Match temperature and other important chemistry during large changes.

8. Record the refill volume, treatment method and any unusual test result so changes in source water can be recognised over time.

A dealer-level water-change checklist

Common mistakes that cause chlorine incidents

• assuming tap water is safe because humans can drink it;
• testing only free chlorine when chloramine may be present;
• dosing dechlorinator after the refill instead of before or during it;
• using an old dose calculation after the pond volume or product concentration changed;
• trusting an activated-carbon cartridge without testing its outlet;
• running carbon at a much higher flow than it was designed to treat;
• assuming standing overnight removes chloramine;
• assuming a small water change is too diluted to matter;
• forgetting that chloramine dechlorination can leave ammonia;
• failing to monitor ammonia and nitrite after a significant disinfectant exposure;
• allowing an automatic top-up to run without flow limits or treatment-failure safeguards.

What to do if untreated tap water accidentally enters the pond

Treat this as a water-quality emergency, especially if the exposure volume is large or koi show respiratory distress.

1. Stop the untreated water source immediately.

2. Add an appropriate chlorine/chloramine neutraliser according to its label and the actual system volume/exposure. If chloramine is involved, use a product or management plan that also addresses the ammonia consequence.

3. Increase aeration aggressively because damaged gills have less respiratory reserve.

4. Test free and total chlorine immediately, but remember that a later negative result does not rule out an earlier exposure.

5. Check dissolved oxygen, TAN, nitrite, pH and temperature.

6. If a safe, fully dechlorinated and temperature-compatible water supply is available, a controlled water change can dilute remaining contaminants and by-products.

7. Do not add unnecessary parasite or antibacterial medications while the fish are already suffering chemical gill injury.

8. Continue ammonia and nitrite monitoring for several days because the biofilter may have been damaged.

9. Seek aquatic-veterinary support for severe gill injury, continuing respiratory distress or substantial mortality.

Why strong aeration matters after exposure

Aeration does not reverse chlorine injury, but it supports fish whose damaged gills are struggling to extract oxygen. It also supports the biofilter if the system has been chemically stressed.

Keep dissolved oxygen high while the pond recovers. A fish with injured gills has less tolerance for marginal DO than a healthy koi.

Do not confuse chemical gill injury with parasites

Koi exposed to chlorine can flash, produce excess mucus, breathe rapidly and crowd near aeration - signs that overlap strongly with parasite problems.

If those signs appear immediately after a refill, investigate source-water disinfectant first. Starting strong parasite medication without checking water chemistry can add another stressor to already damaged gills.

If signs persist after the water is safe, fresh microscopy and professional examination may still be appropriate because chemical injury and infectious disease can coexist.

What if chlorine is already zero but the koi still look distressed?

A zero test after an incident does not prove no exposure occurred. Chlorine may already have reacted with organic material or been neutralised while the tissue damage remains.

Continue evaluating dissolved oxygen, nitrite, ammonia, carbon dioxide and gill condition. Consider the timing of the water change, the amount of untreated water added and whether the biofilter later shows signs of damage.

A practical source-water monitoring table

Common chlorine and chloramine myths

The five rules worth remembering

• Zero detectable free and total chlorine is the goal whenever koi are exposed to municipal water.
• Test both free and total chlorine so chloramine is not missed.
• Treat the incoming water before or as it enters the fish system - never after exposure has already occurred.
• When chloramine is neutralised, think about the ammonia that may remain and whether the biofilter can process it.
• Verify the output of carbon filters and automated treatment systems; never trust equipment without residual testing.

Final perspective

Water changes are among the most important tools in koi husbandry, but only when the replacement water is truly safe. Municipal disinfection is designed to protect people and distribution systems, not fish gills.

The safest koi keeper does not rely on smell, old habits or assumptions about the local tap supply. They know whether the water contains free or combined chlorine, use a treatment method appropriate to that chemistry, verify the result, and understand that chloramine can introduce an ammonia load even after the disinfectant has been neutralised.

Never judge replacement water by how clear it looks. Judge it by what you have measured and what you have removed.

For a home pond, that approach prevents avoidable accidents. For a dealer or high-value collection, it should be standard operating procedure every time new municipal water enters the system.