Untreated tap water in the tank: chlorine, chloramine and the first hour

Quick answer

The fastest way to lose a whole tank is a water change made with untreated tap water. Chlorine and chloramine are put into drinking water specifically to destroy bacteria, and a fish's gill is a sheet of tissue a few cells thick with an enormous surface area held directly in that water.

Fish do not swallow the poison. They breathe it, across the one organ they cannot protect, and the damage starts within minutes.

• Dose the dechlorinator for the whole tank volume, not for the volume of water you added.
• Increase aeration immediately. Damaged gills need every bit of dissolved oxygen available.
• Letting water stand overnight does not remove chloramine. That advice belongs to a different era of water treatment.
• Shrimp and snails die first. If invertebrates crashed and fish are gasping, suspect the water, not a disease.
• Expect an ammonia and nitrite rise over the following fortnight, because chlorine kills the filter bacteria too.

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Never top up or change water straight from the tap into a stocked tank.

Chlorine is a powerful oxidiser and the gill epithelium is one of the most exposed and delicate tissues in any pet animal. The fish suffocates in water that is fully saturated with oxygen, because the surface that transfers it has been destroyed. Fish that survive the first hour frequently die days later from secondary bacterial infection of the damaged gill.
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Decide in 60 seconds

Why the gill is the target

A gill is built to maximise contact between blood and water. Filaments carry rows of lamellae, and each lamella is a flattened sheet only a few cells thick, so that oxygen has the shortest possible distance to travel. The total surface area is very large relative to the size of the fish.

That design is superb for respiration and it offers no protection at all against a dissolved oxidising chemical. Chlorine attacks the lamellar epithelium directly. The tissue swells, cells die, and neighbouring lamellae fuse together, which collapses the surface area available for gas exchange.

The result is a fish suffocating in well oxygenated water. Increasing aeration helps because it raises the concentration gradient across whatever functional gill surface remains, but no amount of oxygen compensates for a gill that has lost most of its exchange area.

Damage also does not stop when the chemical is neutralised. Injured gill tissue is an open door for bacteria, and losses over the following week from secondary infection are common in tanks that appeared to recover.

Chlorine and chloramine are not the same problem

Water companies use either free chlorine or chloramine, and the difference determines what you must do.

Free chlorine is volatile. It escapes from standing water and it can be driven off faster by aeration. This is the origin of the old advice to let a bucket of tap water stand overnight.

Chloramine is chlorine bonded to ammonia. It was adopted precisely because it is stable and persists through long pipe runs, which is exactly what makes the standing bucket useless. It does not gas off in a day, or in several.

There is a second consequence. When a water conditioner breaks a chloramine molecule apart, the ammonia is released into the tank. A product that neutralises chlorine but does nothing about ammonia therefore trades one toxin for another. Use a conditioner that states it detoxifies chloramine and ammonia, and dose for the whole tank rather than for the added volume.

You often cannot tell which your supply uses by smell or by looking. Ask your water company, or check the water quality report they publish. It is worth knowing before an emergency rather than during one.

The other things that arrive with tap water

Copper.

Dissolved from copper plumbing, particularly from new pipework and from water that has stood in the pipes overnight. Copper is highly toxic to invertebrates, so shrimp and snails die at concentrations fish tolerate. Running the tap for a while before collecting water reduces it.

Softened water.

Domestic ion exchange softeners replace calcium and magnesium with sodium. That water is not soft in the sense an aquarist means, it is chemically different, and it is unsuitable for most tanks. Take aquarium water from an unsoftened tap.

Hot water.

The hot supply carries more dissolved metal and more sediment, and in many homes it sits in a cylinder. Mix to temperature using mostly cold water and a kettle, or use a thermometer and a slow blend, rather than drawing from the hot tap.

Temperature shock.

A large volume of water even a few degrees away from tank temperature is a physiological insult on its own, and it looks similar to chemical exposure. Match the temperature before the water goes in.

Detergent.

A bucket that has been used for household cleaning retains detergent that rinsing does not fully remove. Keep one bucket that is used for nothing else, and mark it.

What to do in the first hour

Stop adding water.

Dose a conditioner for the full tank volume. Not for the amount you added. This is the single most important correction to the way most people dose, and it costs nothing.

Increase surface agitation and aeration. Add an air stone, point the filter return at the surface, or lower the water level slightly so the return breaks the surface. Maximise dissolved oxygen.

Do not chase it with another large water change unless you have safe stored water at the right temperature. Another change with the same source repeats the exposure.

Test the water. Ammonia, nitrite, nitrate, pH and temperature. This tells you whether you are now dealing with an ammonia problem from broken chloramine, or a temperature problem, or something that was already wrong.

The test kit is the diagnostic instrument here. Guessing at what happened is what turns one bad hour into a bad fortnight.

Do not add medication, salt, or a general tonic. None of them treat chlorine exposure, several stress a fish with damaged gills further, and adding an unknown makes it impossible to interpret what happens next.

Reduce feeding for several days. Less food means less waste, and a tank whose filter has just been sterilised cannot process ammonia.

Leave the lights off and keep the room quiet. A fish with compromised gills should not be swimming hard.

Only move fish if the tank water itself is unusable, for example after a detergent spill. Netting and transferring a fish that is already in respiratory distress carries its own risk, and treated water in the existing tank is usually the better option.

The second problem: the filter is dead

Chlorine does not distinguish between the bacteria in a water main and the bacteria in your filter. A significant exposure sterilises the biological media, and the tank loses its ability to convert ammonia to nitrite and nitrite to nitrate.

That is why a tank can look stable a day after the incident and then produce sick fish a week later. Ammonia rises because nothing is processing it, and ammonia damages the same gills that are already injured.

Keep one bucket that is used for nothing else, a spare bottle of conditioner, and enough treated water stored to do a change without touching the tap.

For the following two weeks: test ammonia and nitrite daily, feed lightly, do small partial water changes with properly treated water whenever ammonia or nitrite is detectable, and avoid adding any new fish. A commercially prepared bacterial starter can shorten the recovery, but it does not replace testing.

Telling this apart from the things that look like it

Low oxygen from heat or a power cut.

Gasping at the surface, all fish affected, but no recent water change and usually a warm room or a stopped filter.

Ammonia or nitrite poisoning.

Gasping, red or dark gills, lethargy, sometimes cloudy eyes, and a test kit that shows it. Common in new tanks and after a filter has been disturbed or cleaned in tap water.

Gill parasites.

Flashing, rubbing, one gill cover held open, breathing hard, often with the tank otherwise stable and only some fish affected. Develops over days, not minutes.

Bacterial gill disease.

Progressive, often follows a stressor, and usually one or a few fish rather than the whole tank at once.

Carbon dioxide in a planted tank.

Fish at the surface in the evening after a day of injection, resolving overnight. Check the injection rate and surface agitation.

The distinguishing feature of a chlorine event is simultaneity. Every fish is affected at the same moment, immediately after water went in, and invertebrates suffer worst.

Preventing it

What never to do

Do not top up an evaporating tank straight from the tap because it is only a small amount. Small volumes still carry chlorine, and chloramine does not disperse.

Do not rely on letting water stand.

Do not use a stress coat product or a general tonic instead of a dechlorinator. They are different products with different jobs.

Do not do a second large change from the same source to fix the first.

Do not add medication for a fish that is gasping after a water change.

Do not use hot tap water to reach temperature.

Do not use water from a softener.

Do not restock or add anything for at least a fortnight while the filter recovers.

When to get help

Contact an aquatic veterinarian or an experienced aquatic specialist the same day if fish are still breathing hard after conditioner and aeration, if gills look pale, bright red or ragged, or if fish are dying in sequence over hours.

Act immediately, without waiting for advice, on the dosing and aeration steps. They are safe, and delay costs gill tissue.

A tank that comes through this looks unremarkable again within a fortnight. Getting there depends on what you do in the first hour.

Seek advice in the following days for fish that develop white or fluffy patches, frayed fins, cloudy eyes or lethargy, which suggest secondary infection of damaged tissue.

Have ready: the tank volume, the source and temperature of the water added, which conditioner you used and at what dose, the time the exposure happened, your ammonia, nitrite, nitrate and pH readings, the species affected and the order in which they were affected, and whether invertebrates were present. That sequence usually identifies the cause without any further testing.