Chlorine in fish tank: forgot water conditioner? What to do now (chlorine and chloramine)

If you added tap water without conditioner, add water conditioner dosed for the whole tank right now. City tap water carries chlorine or chloramine, and both can harm fish within minutes because they enter the bloodstream directly through the gills (Tampa Bay Water).

Most utilities disinfect with either chlorine or chloramine (chlorine bonded to ammonia, which lasts longer in the pipes). It's fine for you to drink, because your digestion neutralises it. The Merck Veterinary Manual says harm to fish can start at chlorine levels as low as 0.02 mg/L, with deaths at 0.04 mg/L. Tap water usually carries far more than that. Chlorine also kills the helpful bacteria in your filter, which can lead to an ammonia spike days later.

How to identify it

Chlorine poisoning usually follows a water change within minutes to hours. Look for:

Fish darting, twitching or rubbing against objects right after the water change.
Fast breathing, gasping at the surface.
Pale or blotchy colour; fish hanging at the top or lying on the bottom.
After milder exposure: excess slime on the skin, cloudy eyes, ragged fins, red or damaged gills (Merck).

Easily confused with: temperature shock from water that was too hot or cold; soap or other contaminants in a bucket; a pH swing; and ammonia. The timing (right after a water change) is the strongest clue. A chlorine test kit or strip can confirm it, but Merck warns chlorine is short-lived, so a negative test after the fact doesn't rule it out.

When it's a problem

Early: fish darting, twitching or breathing fast in the minutes to hours after a water change. Most recover quickly if you dose conditioner and the exposure was short.
Serious: fish still gasping an hour after dosing, or slime, cloudy eyes and damaged gills showing up. An ammonia rise a few days later, as filter bacteria die off.
Emergency: a full tank of untreated chlorinated water, or many fish dying. This can kill fish within hours.
Normal, don't worry: treated tap water, dosed correctly, with calm fish afterwards.

What to do right now

Add water conditioner now, dosed for the whole tank volume, not only the water you added. Use one that says it handles chloramine and, ideally, binds ammonia. Conditioners act within minutes. A modest overdose of a quality conditioner is generally safe; follow the label for the maximum.
Add strong aeration: an air stone, or the filter outlet breaking the surface.
Watch the fish for 30 minutes.
Don't do another big water change with untreated water. If you do more changes, treat the new water first in a bucket.
Test ammonia and nitrite daily for a week. The filter bacteria may have been hit, so a mini-cycle can follow (see cycling).
Feed lightly for a few days while the filter recovers.

What to do next

Chlorine vs chloramine

Chlorine slowly escapes from water left standing for a day or more, especially with aeration.
Chloramine does not. It can take weeks to dissipate. Tampa Bay Water says plainly that letting water sit is "not a safe method" for chloramine. Boiling isn't a practical fix either.
When chloramine is neutralised, the chlorine part is destroyed but the ammonia part is released. Merck notes that repeated water changes with chloramine water can push ammonia to toxic levels unless the conditioner also binds ammonia.

Many utilities, including many in Canada and the US, now use chloramine. Check your water utility's website or annual water quality report, or call them.

Neutralise

Most dechlorinators use sodium thiosulfate or similar chemicals that destroy chlorine and break chloramine instantly. Some add an ammonia binder that holds the released ammonia in a less toxic form for about a day, while your filter bacteria use it up. Follow the label for your product; strengths vary a lot.

Experts disagree on dosing details. Some products tell you to dose for the whole tank at every water change; others say to dose only for the new water. Aquarium Science argues that only the new water needs treating, and that some conditioners' ammonia claims are overstated. In an emergency, treating the whole tank is the cautious choice.

Support recovery

Aeration helps damaged gills.
Keep the water extra clean for 1–2 weeks. Damaged skin and gills are open to infection; watch for fin rot and fungus.
Protect the filter. Don't clean it now. If ammonia rises, do small conditioned water changes and consider a bottled bacteria product.
No medicine is needed for chlorine itself. Only treat infections if they appear.

Cautions

Shrimp, snails and other invertebrates are extremely sensitive. Tampa Bay Water lists shrimp, snails and frogs among affected animals.
Scaleless fish, plecos and loaches often show damage sooner.
Plants usually survive short exposures.
Don't combine conditioners that contain unknown ingredients with formalin-based medicines; some conditioners contain additives that interact.

Products that help

Water conditioner that handles chloramine and binds ammonia: neutralises chlorine and chloramine within minutes and holds the released ammonia for about a day. Use at every water change, and dose the whole tank in an emergency.
Air stone and air pump: extra oxygen for damaged gills.
Ammonia and nitrite test kit: daily for a week after an accident, to catch a mini-cycle.
Chlorine test kit or strips: can confirm chlorine in the water, though a negative test after the fact doesn't rule it out.
Bottled filter bacteria: worth considering if ammonia rises after the filter was hit.

Before trouble starts, keep a spare bottle of conditioner. In real life it costs a few dollars and is the cheapest insurance in the hobby.

How to prevent it

Find out what your utility uses: chlorine or chloramine.
Keep a spare bottle of conditioner. Running out is how most accidents happen.
Make it a habit: conditioner in the bucket before the water. With a direct-fill hose, dose the tank for its full volume before you start refilling.
Do routine changes of 20–30% rather than huge ones, so a mistake is smaller.
Sign up for your utility's notices, if they offer them.

Why it happened

Forgot the conditioner, or ran out.
Filled the tank straight from a hose or tap without dosing the tank first.
Used a "chlorine only" product on chloramine water.
Assumed letting chloramine water stand overnight was enough.
Your utility temporarily switched disinfectant or raised the dose (they sometimes do this for maintenance, often with a public notice).
Used a home water filter or softener that doesn't remove chloramine.

When to get help

Get help if fish are still gasping an hour after dosing conditioner, if many fish have died, or if your tap water tests positive for ammonia, nitrite or copper. A good local fish store can test your tap water and recommend a conditioner for your city. A vet who sees fish can help with gill damage and infections afterwards. Honest limit: a full tank of untreated chlorinated water can kill fish within hours, and you may not save all of them.

In the game

In Multiple Tank Syndrome, the Chlorine card is "a water change straight from the tap, no conditioner": the whole tank is lost. The Water conditioner card saves it. In real life, a bottle of conditioner costs a few dollars and is the cheapest insurance in the hobby.